The gene SSK1 is found on chromosome 7 in Saccharomyces cerevisae. It is named for its role as a suppressor of sensor kinase. The protein it encodes is a cytoplasmic response regulator; it is part of a osmosensory system that regulates an mitogen-activated protein kinase cascade. This MAP kinase cascade controls the concentration of glycerol in yeast cells experiencing hyperosmotic stress (Maeda, 1994). A search of the RCSB Protein Data Bank yielded no stuctures for SSK1 (PDB, 2004).


Figure 1: Chromosomal location of SSK1. This diagram shows a region of Saccharomyces cerevisae chromosome 7 from the 151754 to the 173892 bp. The gene SSK1 is highlighted in red (SGD, 2004).

The molecular functions of SSK1 are enzyme activation and two-component response regulation. The protein product of SSK1 is found in the cytoplasm, where it interacts with the proteins SLN1 and YPD1 as part of a two-component osmosensor. Under normal osmotic conditions, phosphorelay from SLN1 to YPD1 to SSK1 keeps an aspartic acid residue in the receiver domain of SSK1 phosphorylated (Janiak-Spens, 1999). In this phosphorylated state, SSK1 is inactive. Under hyperosmotic stress conditions, however, SSK1 is dephosphorylated through either suppression of SLN1's kinase ability or the involvement of an as-yet unidentified phosphatase. The dephosphorylated SSK1 activates SSK2 by binding to and releasing its N-terminal autoinhibitory domain; SSK2 then induces the HOG1 MAP kinase cascade to raise the level of glycerol within the cell and restore osmotic balance (Posas, 1998). Schematic diagrams of this osmoregulatory pathway are shown in figures 2 and 3. The two-component system also responds to oxidative stress induced by hydrogen peroxide and diamide. SSK1 activates SSK2 to start the MAP kinase cascade upon sensing oxidative stress from either oxidant (Singh, 2000).

Figure 2: Osmoregulatory signal transduction pathway in Saccharomyces cerevisae. The two-component phosphorelay system composed of SLN1, YPD1, and SSK1 negatively regulates the MAP kinase cascade. SHO1 is an alternative osmosensor. The arrows do not necessarily indicate direct interactions (Posas, 1998).

Figure 3: Alternate diagram of phosphorelay in Saccharomyces serevisae osmoregulation. Under hyperosmotic stress conditions, dephosphorylation of SSK1 results in activation of the MAP kinase cascade, which raises the level of glycerol within the cell to restore osmotic balance. Solid bars indicate transmembrane regions (Janiak-Spens, 1999).

As indicated above, SSK1 is known to be involved in two biological processes, osmoregulation and the response to certain oxidants. As a part of the two-component phosphorelay system, SSK1 regulates the response to hyperosmotic stress by controlling the signaling pathway that activates the MAP kinase cascade and leads to increased glycerol production, thus returning the cell to osmotic balance (Maeda, 1994). SSK1 also regulates yeast cells' response to oxidative stress from hydrogen peroxide or diamide by activating the same MAP kinase cascade to minimize damage to the cell (Singh, 2000). As essential as SSK1 might sound, however, figure 4 shows that the lack of SSK1 causes only a moderate growth defect. SSK1 null mutants are viable, although they exhibit sensitivity at 15 generations when grown in a medium of pH 8. SSK1 null mutants actually benefit from their mutation if they also have disruptive mutations in the genes encoding SLN1 or YPD1. These two proteins negatively regulate SSK1; without them, SSK1 and the MAP kinase cascade remain permanently activated. The lack of functional SSK1 therefore suppresses the lethality of such mutations (SGD, 2004).

Figure 4: Fitness score for SSK1 vs. the probability of its being essential. After growth on YPD for 20 generations, yeast cells with non-functional SSK1 experience only an intermediate growth defect, indicating that SSK1 is important but not absolutely essential for growth (SGD, 2004).

The gene YLR012C is found on chromosome 7 near SSK1 in Saccharomyces cerevisae. It encodes a protein whose molecular functions, biological processes, and cellular components are unknown (SGD, 2004). YLR012C null mutants are viable, with no growth defects detected after 20 generations on YPD, as seen in figure 6. It does not appear to be an essential gene. A BLASTp search for YLR012C's amino acid sequence yielded only one hit other than the unknown Saccharomyces cerevisae protein itself; this protein is an unknown protein from Eremothecium gossypii, another species of yeast (NCBI, 2004). The BLASTp search also failed to reveal any conserved domains, so as of now this protein is a complete mystery.

Figure 5: Chromosomal location of YLR012C. This diagram shows a region of Saccharomyces cerevisae chromosome 7 from the 159981 to the 180280 bp. The gene YLR012C is highlighted in red (SGD, 2004).

Figure 6: Fitness score for YLR012C vs. the probability of its being essential. After growth on YPD for 20 generations, yeast cells with non-functional YLR012C experience no growth defects, indicating that YLR012C is not an essential gene (SGD, 2004).

Figure 7: BLASTp results from YLR012C. The only other hit is the protein AGR353Cp, located on chromosome 7 in Eremothecium gossypii (NCBI, 2004).

Janiak-Spens, F., et al. 1999. Differential stabilities of phosphorylated response regulator domains reflect functional roles of the yeast osmoregulatory SLN1 and SSK1 proteins. Journal of Bacteriology 181(2): 411-417. <http://jb.asm.org/cgi/content/full/181/2/411?view=full&pmid=9882653> Accessed 2004 Oct 8.

Maeda, T., Wurgler-Murphy, S.M., and Saito, H. 1994. A two-component system that regulates an osmosensing MAP kinase cascade in yeast. Nature 369(6477): 242-245. <http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=8183345> Accessed 2004 Oct 6.

National Center for Biotechnology Information (NCBI). 2004. <http://www.ncbi.nlm.nih.gov/> Accessed 2004 Oct 8.

Posas, F., and Saito, H. 1998. Activation of the yeast SSK2 MAP kinase kinase kinase by the SSK1 two-component response regulator. The EMBO Journal 17: 1385-1394. <http://embojournal.npgjournals.com/cgi/content/full/17/5/1385> Accessed 2004 Oct 6.

RCSB Protein Data Bank (PDB). 2004. <http://www.rcsb.org/pdb/index.html> Accessed 2004 Oct 6.

Saccharomyces Gene Database (SGD). 2004. <http://www.yeastgenome.org/> Accessed 2004 Oct 8.

Singh, K.K. 2000. The Saccharomyces cerevisiae sln1p-ssk1p two-component system mediates response to oxidative stress and in an oxidant-specific fashion. Free Radical Biology and Medicine 29(10): 1043-1050. <http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=11084293> Accessed 2004 Oct 8.