For Nov. 26, 2001 last modified Nov. 26 8AM
Evolution of Signal Transduction
David Nelson
Dr. Fains next set of lectures will be on signal transduction. This involves a cell
responding to a stimulus. Both proteins and small molecule second messengers are key
players. This lecture will deal with the evolution of these pathways. Where did they
arise, and how far back on the tree of life do they exist.
There are a variety of ways to transmit a signal across a cell membrane
and into the interior of the cell, or even into the nucleus. Many of these pathways use
the same components, so these may be viewed as modules of the signal transduction
process. A list of these components is given below.
1) Seven transmembrane segment receptors (also called serpentine receptors) coupled to
heterotrimeric G proteins. G-protein coupled receptors (GPCRs)
2) Single transmembrane segment receptors (receptor tyrosine kinases, receptor guanylyl
cyclases, histidine kinases and two component regulators)
3) Nuclear Receptors
4) Ion channels, either voltage gated, ligand gated or mechanosensitive
A Link to ion channel resources for more information on ion channel structure, function and evolution.
5) calcium/ calmodulin
6) phospholipases/ sphinomylinase and products diacylglycerol, phosphoinositides and
ceramide
7) Nitric oxide synthase and soluble guanylate cyclases
8) cyclic nucleotides cAMP, cGMP, adenylate cyclase and phosphodiesterases
9) protein kinase cascades and associated phosphatases
Lets take these in turn and look at the evolutionary history of these systems.
On the tree of life, plants, animals and fungi branch from nearly the same point, about
1.2 billion years ago. This tight cluster of branches is called the crown group of eukaryotes.
Plants and several groups of protists branched off first followed by Amoebozoa
(includes the cellular slime mold Dictyostelium). Fungi and animals
diverged soon after. Dictyostelium discoidium has been well studied because it
assembles from single cells to form a slug that develops into a fruiting body that makes
spores. Of course, this involves signal transduction, so we will look at some signal
transduction pathways in Dictyostelium as a representative primitive eukaryote. We will
also consider yeast, since the whole genome is sequenced and it has been thoroughly
studied as a model organism. In 1998, C. elegans the nematode worm had its genome
sequence completed. In March 2000 the Drosophila genome was placed in Genbank.
On Dec. 14 2000 the Arabidopsis genome was published in nature. These genomes will offer
insights into signal transduction in animals and plants but it may take years to analyze
all the gene data.
Seven Transmembrane Receptors
Seven transmembrane segment receptors are eukaryotic proteins. Animals are
especially fond of these receptors. The genome of the nematode worm is now done
and it contains 1049 G-protein coupled receptors. These may be chemosensory
receptors. Almost all of these are orphan receptors without known ligands. In fact,
only one ligand has been linked to its chemoreceptor in C. elegans and that is diacetyl.
This ligand binds to the odr-10 gene product that normally mediates attraction to
diacetyl. However, if the gene is expressed in another neuron that usually mediates
repulsion from a substance then the worm is repelled by diacetyl. This shows that the
receptor only triggers a pathway and the response is dependent on which neuron the gene
is expressed in. The worms genome has 18,424 genes total, so 1049 odor receptors is
about 5.7% of the worms genome dedicated to chemosensory receptors. Current estimates
suggest that 300 of these genes are pseudogenes and about 200 are not expressed in
neurons, so they are not likely to be chemoreceptors, but that still leaves 500
serpentine receptor genes that are probable functioning chemoreceptors. This is the
largest family of genes in the worms genome. (see Seven-transmembrane proteins as
odorant and chemosensory receptors. Science. 1999 Oct 22;286(5440):707-11, and
Chemosensory signaling in C. elegans. Bioessays. 1999 Dec;21(12):1011-1020.)
Even yeast have examples of these receptors, but not nearly as many. In a brief search,
I found only 3: STE2, STE3 and GPR1. These are receptors for pheromone (STE2 and STE3)
and a receptor that is induced during starvation. The ligand for GPR1 is glucose which
causes a cAMP spike in starved yeast cells(Mol. Microbiol. 38, 348-358 2000).
The heterotrimeric G protein alpha subunits for these receptors are named GPA1
(for STE2 and STE3) and GPA2 (for GPR1). There only appear to be two G protein alpha subunits
in yeast, and two in the fission yeast S. pombe. Based on these results, the fungi have not
exploited these useful receptors very much. They seem to be used for mating purposes and for
sensing the concentration of glucose in the environment. Dictyostelium also uses them for cell
to cell communication in the developmental program of aggregation and sporulation.
Dictyostelium branches on the eukaryotic lineage within the crown group of
eukaryotes (animals, fungi, amoebozoa, plants, stramenopiles and alveolates). The signal
transduction mechanisms present in Dictyostelium should be present in animals and fungi,
but not necessarily in plants. There are genome projects being done now that include
Dictyostelium and Giardia, one of the most distant eukaryotic organisms known. These
projects should aid greatly in understanding the earliest events in eukaryotic evolution,
including development of signal transduction pathways. But the genomes are not done yet,
so the story is fragmentary so far.
Searches for seven transmembrane receptors and G-proteins in Dictyostelium show
that there are four cAMP receptors called cAR1 to cAR4. These are able to sense cAMP
secreted by the Dictyostelium cells as they aggregate to form a stalk. These four
receptors are expressed at different times during the developmental program of the
organism. However, one report (EMBO J 1998 Sep 1;17(17):5076-84) refers to deletion of
the single G-protein beta subunit. There appears to be only one beta subunit that is
shared by all four receptors. The situation is not the same for the alpha subunits,
since there is sequence data for at least eight distinct alpha subunits. It is not clear
if there are eight different seven transmembrane receptors for all these alpha subunits,
or if the four cAR receptors use more than one, possibly at different times.
One interesting finding concerning these serpentine receptors in Dictyostelium is
that some responses are not dependent on the beta subunit. These seem to be G-protein
independent responses that are linked to the receptor. One of these is calcium influx,
though the amount of calcium influx is less than with the intact G-proteins. Here we may
be seeing a remnant of early evolution of the serpentine receptor/G-protein couple. At
one point in the past there had to be a receptor without G-proteins like the
bacteriorhodopsin light driven pump discussed below. If the cAMP receptors can carry out
some of the same functions, but at a reduced efficiency, then this might be a vestige of
the original receptor function without G-proteins. A related issue is activation of
receptors without ligand bound. The Nov. 98 TIBS has an article on Agonist Independent
Regulation of Consitutively Active G-protein Coupled Receptors. Constitutive signaling
without ligand bound has been documented now for several GPCRs.
The seven transmembrane receptors have a remarkable similarity to
bacteriorhodopsin and halorhodopsin. These are a light driven proton pump and a light
driven chloride pump from the archaebacteria Halobacterium salinarium (previously named
halobium). The seven transmembrane segments enclose a retinal attached to a lysine
residue of the last helix by a schiffs base. This retinal undergoes an all-trans to 13-
cis isomerization when it absorbs a photon. The conformational change is transmitted to
the protein and it is used to pump protons or chloride and form a gradient. In
vertebrate eyes, the protein rhodopsin is also a seven transmembrane segment protein with
retinal attached to a lysine also in the last helix. In this protein the conformational
change is 11-cis to all trans and it activates a heterotrimeric G protein that activates
a cGMP phosphodiesterase which reduces cGMP concentrations. The result is closing of
sodium ion channels and electrical stimulation of a nerve.
The common ancestry between bacteriorhodopsin (BR) and eukaryotic receptors
might be considered unconvincing if the retinal attachment and seven transmembrane
segments were the only evidence in its favor. However, the relationship is supported based
on more detailed evidence. BR has two close relatives in Halobacterium salinarium that
are light driven sensory receptors SRI and SRII. These also contain retinal, but they
are tightly coupled to two transducer proteins HtrI for SRI and HtrII for SRII. If the
sensory rhodopsins are expressed without the transducers they function as proton pumps.
The tight association between the sensory rhodopsins and the transducer prevents proton
pumping. Instead, the conformational change caused by light is used to stimulate the
transducers. These proteins are histidine kinases that control a phosphorylation cascade that
regulates the flagellar motor as in E. coli chemotaxis. Different wavelengths of light can
either attract or repel the bacteria. In this way, the transducer proteins are similar to
two component regulators described below.
The most convincing evidence for common ancestry of bacterial and eukaryotic
rhodopsins is in the mechanism of signal transduction. Both bacterial and vertebrate
rhodopsins form a protonated schiffs base on the G helix where retinal is attached. Both
form a salt bridge between this protonated schiffs base and a conserved carboxyl group on
helix C. Absorbance of a photon breaks the salt bridge and shifts the proton to the
carboxyl group. In BR the net result is opening of a half channel facing the cytoplasm
and transport of 2 H+ across the membrane. In sensory rhodopsin, the signal is
transmitted to the Htr transducer protein and on into the cell by phosphorylation. In
eukaryotes, the trimeric G proteins separate into alpha and (beta, gamma) subunits.
There is no sequence similarity between bacteriorhodopsin and vertebrate rhodopsin.
However, the overall structure and the placement of retinal are two pieces of evidence that
suggest a common ancestor. If this is correct, then the archaebacterial ancestor of eukaryotes
may have contributed a seven transmembrane precursor of modern seven transmembrane receptor
proteins. J.L. Spudich, an expert in these proteins does not believe that archaeal and
visual rhodopsins are descended from a common ancestor. He argues that the 22 amino acids
that make up the binding pocket for retinal are not conserved in the vertebrate rhodopsins
and the retinal structure is significantly different. High resolution crystal structures are
available now for bacteriorhodopsin trapped in various stages of the transport cycle.
(See Science 286, Oct. 8, 255-260 and 252 commentary 1999); halorhodopsin (Science 288 1390-1396
2000) and bovine rhodopsin (Science 289, 733-734, 739-745 2000) The interpretation of the
structures suggests that the seven helices are arranged in different ways. The consensus
seems to be that the vertebrate rhodopsins (and therefore all the GPCRs) are not related to
the archaeal rhodopsins (Curr. Opin. Struct. Biol. 11, 420-426 2001).
Therefore, the origin of seven transmembrane segment receptors cannot be traced back to
light sensitive receptors in archaebacteria. The similarity between these proteins seems
to be a spectacular case of convergent evolution. A new development in 1999 is the
discovery of a fungal protein called nop-1 (opsin from Neurospora crassa) that has been
overexpressed in Pichia pastoris. The protein binds retinal as a schiff base and
probably acts as a rhodopsin like molecule in N. crassa (PNAS 96, 8034-8039 1999) This protein
has 20 of 22 conserved amino acids in common with archaeal bacteriorhodopsin. There
are other related proteins in N. crassa that do not have the lysine residue that makes
the schiff base, so these may have lost the ability to use retinal as a stimulus, but
this is expected in the evolution of the seven transmembrane receptors that do not
contain retinal and are driven by ligand binding. (for reviews search for JL Spudich).
At the current time the origin of the seven transmembrane receptors is not clear.
A 1998 report claims to have found the first seven transmembrane receptor in plants
(Plakidou-Dymock S, Dymock D, Hooley R. A higher plant seven-transmembrane receptor that
influences sensitivity to cytokinins. Curr Biol 1998 Mar 12;8(6):315-24). The Arabidopsis
genome published in Dec. 2000 shows 27 GPCR domains present, so these receptors
are not common in plants. Additional support for GPCRs in plants comes from the fact that
Arabidopsis has a single G-protein alpha subunit GPA1 (Curr. Biol. 11, R869-R874 2001).
Knockout of the alpha subunit in rice causes a dwarf phenotype and abnormal seeds (Plant
Phisiol. 42, 789-794 2001). These GPCR proteins are then found in four main
branches at the top of the eukaryote tree, animals, fungi, amoebozoa and plants.
Beyond that no sequence data is available for GPCRs in the lower eukaryote groups.
Single Transmembrane Segment Receptors
Some receptors with single transmembrane segments unite as dimers and have
enzymes associated with them on the cytosolic side of the membrane. These enzymes can
be tyrosine kinases, serine/threonine kinases, tyrosine phosphatases, guanylyl cyclases
that form cGMP or histidine kinases. The histidine kinases are ancient signal
tranduction molecules found in all three domains of life. These are called two component
regulators. This system is used in bacterial chemotaxis, where a receptor binding ligand
activates a histidine kinase CheA to phosphorylate itself. The PO4 group is rapidly
transfered to an Asp residue on another soluble protein CheY called the response
regulator. This protein then carries out some regulatory function. In chemotaxis CheY
binds to the flagellar motor and influences the direction it turns. Clockwise rotation
causes tumbling which is a kind of retreat from a repellant substance. Counterclockwise
rotation makes the E. coli swim smoothly ahead. CheY that is phosphorylated causes
clockwise rotation. The HtrI and HtrII proteins of Halobacterium are similar to CheA,
except they have two transmembrane segments instead of one.
Dictyostelium has a his kinase response regulator system. The receptor DhkA
binds a secreted peptide SDF2 that triggers autophosphorylation on histidine. The PO4 is
transfered to RegA, a cAMP phosphodiesterase which is inhibited. The increase in cAMP
activates protein kinase A (PKA).
Yeast also has one of these response regulatory his kinase receptors for sensing
osmotic stress. It feeds into the Hog1 MAP kinase pathway (see below).
Receptor tyrosine kinases are only found in metazoa[animals] (J. Mol. Evol. 44, 242-252
1998). One has been cloned from a sponge, which is as far back as one can go in the
metazoa. This gene is interesting because it contains only two introns. More advanced
animals have more introns in their tyrosine kinase genes. Since the sponge gene appears
to branch on a tree before the other metazoan receptor tyrosine kinases, this suggests
that introns were added after this branching occurred, favoring the introns late theory.
Nuclear Receptors
Nuclear receptors were called the largest family of transcription factors in a
1994 Annual Rev. of Biochem. article (vol. 63, p. 453). In a Nov. 98 article, more than
300 sequences were known (PNAS 95, 13442-13447). These are internal receptors in contrast
to the membrane receptors we just discussed. The ligands for these receptors are
hydrophobic and diffuse through the membranes of cells without specific transport
proteins to bring them in. They include steroids, retinoids, vitamin D and thyroid
hormone. Vincent Laudet has been active in the study of the evolution of these receptors.
His group has searched for the presence of nuclear receptors in many types of animals,
plants and lower eukaryotes. The approach was an exhastive PCR study using highly
conserved regions of the nuclear receptors to make degenerate PCR primers that could be
used to detect almost any member of the group. The result was that nuclear receptors are
strictly metazoan (animal). They were not found in plants, algae, fungi or protists.
Among animal species, they were not found in sponges, but they were present in Cnidarians
(radial animals like jellyfish, hydras and corals) and all other more advanced animals.
There are 270 nuclear hormone receptors in C. elegans alone.
Many nuclear receptors do not have ligands identified. These are called orphan
receptors. One surprising finding in this exhaustive search for receptors was that a
given ligand did not appear in only one branch of the tree of receptors. Also, the
receptors with known ligands were predominantly found in recenty evolved subfamilies. The
authors concluded that the ability to bind ligand has only evolved recently and the
ancestral nuclear receptors were probably orphan receptors. This also suggests that many
orphan receptors do not have any ligand that binds to them. To function without ligand
the receptor is proposed to have two conformations. One binds to DNA and regulates gene
function. The other does not bind DNA. The equilibrium between the two states
determines how much time the gene is turned on. Binding a ligand shifts the equilibrium
to the DNA binding conformation. Of course other things could shift this equilibrium
too, like phosphorylation, temperature, etc. This idea is new and will have to be
debated and tested. In 1999 a nomenclature system was adopted for nuclear receptors
(A unified nomenclature system for the nuclear receptor superfamily. Cell. 1999 Apr
16;97(2):161-3). Nomenclature is a serious problem as more genomes are sequenced and it
needs to be addressed for most large families of genes.
Ion Channels
Ion channels include neurotransmitter activated channels like the acetylcholine
receptor (ligand gated), voltage gated channels and mechanosensitive channels that
respond to changes in membrane properties that could be altered by mechanical forces
caused by high or low osmolarity conditions or shear forces. The ligand gated channels
like the acetylcholine receptor are more advanced. They are found at synapses and these
are nerve specific. They are not found outside animals.
The mechanosensitive channels are found widely in nature in bacteria,
archaebacteria and eukaryotes. These are the simplest channels, since thay do not need a
ligand binding site or a voltage sensor. They do have to exist in at least two states
(open and closed) with possibilities for additional substates. The nature of channels is
determined by patch clamp techniques, where the size of the channel is estimated from the
current it passes and the duration of open and closed states can be measured. A
mechanosensitive channel has been purified from the archaeon Haloferax volcanii. It was
only 37kDa, much smaller than vertebrate ion channels that can be about 2000 amino acids
and 200kDa.
Voltage gated ion channels include the porins of bacteria, mitochondria and
archaea. However, porins are quite different from the nerve based ion channels for Na, K
and Ca. These channels have alpha helical transmembrane segments, while porins have a
beta barrel structure. The two protein superfamilies must have separate evolutionary
origins. Among the K, Na and Ca channels, the K channel appears to be the oldest. It is
a tetramer of identical subunits, whereas the Na and Ca channels have four domains, each
similar to one K channel. K channels have been found in bacteria, in 1998 a
crystal structure was determined of the Streptomyces lividans K channel. It is
tetrameric, with each identical subunit contributing two transmembrane segments. The
pore is in the center. It made the cover of Science (Vol. 280, 69-77 April 3 issue).
Some eukaryotic K channels have six transmembrane segments rather than two, but the first
four seem to be extra. The last two transmembrane segments are very similar to the
bacterial K channel. The evolution of Na and Ca channels probably started from a K
channel. Recently, a Na channel has been cloned from a jellyfish (Cnidarian). These are
the most primitive organisms with nerves.
Calcium/ Calmodulin
Calmodulin is a calcium binding protein that activates many different enzymes
such as plasma membrane calcium pumps, phosphodiesterases, kinases(CAM kinase II) and
phosphatases (calcineurin). It binds calcium at low micromolar concentrations, changes
its conformation and binds to its target enzymes. Calmodulin is found in every eukaryote
ever looked at, but it is not found in bacteria. A report has been published that a
calmodulin like protein is present in Halobacterium salinarium an archeabacterium. This
protein binds calcium, it activates cAMP phosphodiesterase and is inhibited by calmodulin
inhibitors, but the sequence is not known yet. The Halobacterium genome was completed in
summer 2001, but it is not publicly available yet. A search of the annotation at the genome
website looking for calmodulin did not find any hits.
The structure of calmodulin has four EF hands that are motifs that bind calcium.
The EF hands are numbered I, II III and IV. I and III are more similar to each other
than they are to II and IV. II and IV are more similar to each other than they are to I
and III. The interpretation of this is that the protein evolved from a gene with a
single calcium binding motif that duplicated and diverged for a time then duplicated
again. The original one domain gene is now vanished into the distant past and may not
exist any more, or it will probably be impossible to detect the similarity to it. It is
unfortunate that many evolutionary questions of origins cannot be solved because the
process of evolution has erased the books.
Phospholipases and Sphingomylinase
Phospholipases are active members of signal transduction pathways. PI specific
Phospholipase C (PI-PLC) generates diacylglycerol and inositol triphosphate (IP3), each
of which have effects in the cell. The structures of PI-PLCs of a bacterium and a
eukaryote have now been solved. Though the sizes are quite different, 30-35kDa for
bacteria and 85-150kDa for eukaryotes, the catalytic domain of the eukaryotic protein is
made from distinct parts of the sequence that come together to form a (beta alpha)8-
barrel. The first half of the barrel contains the catalytic conserved amino acids and it
is highly conserved in the structure, but not in sequence. The second half of the barrel
is where the substrate binding pocket is. This part is not as conserved, since the
preferences of the enzymes are different. One difference is a catalytic Ca in eukaryotes
that is replaced by an Arg residue in bacteria. The catalytic domains of both preserve a
similar fold and similar mechanism, therefore, they must be descended from a common
ancestor.
Sphingomylinase cleaves sphingomylin to release ceramide, a mediator of stress
responses. Ceramide activates several proteins including ceramide activated protein
kinase, ceramide activated protein phosphatase and protein kinase C zeta. Ceramide
signaling has numerous effects, but often they end in apoptosis (programmed cell death).
The classical apoptosis pathway does not exist in yeast, yet the ceramide stress response
is found in yeast. Therefore, the linkage of ceramide responses to apoptosis seems to be
acquired after animals diverged from fungi.
Nitric Oxide Synthase
Nitric oxide synthase produces NO radical and citrulline from arginine. The NO is
a signal that rapidly degrades and it cannot be turned off. It also diffuses through
membranes so that no transport mechanism is needed. Its action is short lived and
localized. The C-terminal of the enzyme NOS has 36% sequence identity to cytochrome
P450 reductase. It contains FMN, FAD and a binding site for NADPH. This part of the
molecule must have evolved from cytochrome P450 reductase. The N-terminal has the
heme and the active site where the NO is made. There is a heme thiolate (S-) ligand
which gives the spectral properties of a cytochrome P450 enzyme, but other highly
conserved features seen in all P450s are missing. This part of the enzyme is probably
not derived from a P450 ancestor, but arose from some other source. The thiolate anion
ligand to the heme seems to be a case of convergent evolution. The whole protein must be
the result of gene fusion between the P450 reductase part and some other gene that coded
for the N-terminal half. There are a few reports of NOS activity in insects, plants and
yeast, but the genes for these activities are not identified yet.
Cyclic Nucleotides
cAMP is found in all three domains of life: bacteria, archaea and eukarya. In
eukaryotes, it activates protein kinase A (PKA). It is secreted as a signal by
Dictyostelium and it binds to seven transmembrane receptors in this organism. Bacteria
use it to regulate gene expression in catabolite repression, where glucose can repress
the synthesis of genes needed to metabolize other carbon sources. The glucose signal
activates a cAMP phosphodiesterase that decreases cAMP concentrations. cAMP normally
binds to a protein called CRP (cAMP receptor protein) or CAP (Catabolite gene-activator
protein) and this protein binds to DNA and activates gene expression of an operon. When
the cAMP level drops, the CAP protein can no longer activate the genes and their
transcription is reduced or stopped. cAMP is one of the most ancient of regulatory
signaling molecules.
MAP Kinase cascades
The MAP (mitogen activated protein) kinase cascade is a useful example to follow
back in time. The MAP kinases exist in yeast, and since the whole genome is known, all
the MAP kinases can be identified in yeast. There are six of these genes. (Trends in
Genetics 14, 151-155 1998 and Cell 80, 187-197 1995). They are FUS3, KSS1, HOG1,
MPK1, SMK1 and MLP1. A table showing the actions of these MAPKs is given below.
MAPK external stimulus effect
Fus3p pheromone response mating
Kss1p starvation filamentation
Hog1p high osmolarity osmolyte synthesis
Mpk1p Hypotonic shock cell wall remodeling
Mlp1p unknown possibly similar to Mpk1p
Smk1p carbon and nitrogen deprivation sporulation
All of these kinases are important for some aspect of the yeast life cycle and its
response to its evironment. However, all six of these genes can be deleted in a single
host strain and the cells will live, though they wont be very robust (Cell 91, 673-674
1997). They will not be able to respond to external signals and they have to be treated
pretty gently.
Two of these MAPKs share parts of their signal transduction pathways. This poses
the difficult question of how the final end response is unique when the middle of the
pathway is the same. Fus3p in the mating pheromone pathway and kss1 in the
filamentation pathway share three kinases in common. These are STE20, STE11 and
STE7. These are the yeast equivalents of human PAK, MEKK and MEK. [Note: yeast
mutants that are defective in mating are called STErile mutants and their genes are
numbered as STE#. There are dozens of sterile mutants in yeast]. The solution to this
problem is found in inhibitory functions for both Fus3p and Kss1p. These inhibitory
functions are independent of their kinase activities.
If Fus3p is deleted rather than inactivated by a point mutation, the yeast cells
can still mate. It has been suggested that Kss1p substitutes for Fus3p in this case.
Surprisingly, the filamentation pathway is also activated under these conditions
(addition of pheromone). The interpretation of these results is that Fus3p is inhibitory
to the activation of filamentation and it normally blocks this pathway. If a point
mutation is used in Fus3p, then the inhibitory role of Fus3p is preserved and the
filamentation pathway is not activated by pheromone.
There is one problem with this model. If Fus3p inhibits filamentation, then how
can filamentation ever occur? Somehow this inhibition must be overcome when a
filamentation signal is detected. One way this might happen is by forming complexes of
the shared components with their unique MAPKs. In effect, this would make a unique
complex. A protein called Ste5p has been shown to form a complex with Ste11p, Ste7p
and Fus3p. The complex may make the whole pathways behavior unique even though
there are shared components. The exact details of this complex regulation have not been
worked out. For recent findings see Mol. Cell 8, 683-691 2001.
The sensor of the external conditions for these pathways is not known in all
cases. The pheromone pathway for mating has two seven transmembrane receptors STE2 for
the alpha mating factor and STE3 for the a mating factor. These have a heterotrimeric G
protein. The HOG1 pathway has two independent osmosensors. Neither of these is a
seven transmembrane receptor. One called SHO1 has an SH3 domain in the cytoplasm that
binds to a MAPKK called PBS2. This phosphorylates and activates HOG1. The other
osmosensor is a histidine kinase that also feeds into the PBS2 kinase. The histidine
kinase and its target Ssk1p are similar to a two component regulatory signaling system
seen in bacteria.