Functional analysis of the YGR262c/Vsk1 gene encoding an atypical protein kinase essential for normal cell growth

The S.cerevisiae YGR262c gene, whose deletion confers to yeast cells a pleiotropic phenotype characterized, among other defects, by severe slow-growth, inability of homozygous diploids to enter sporulation and important alterations in cell wall structure, encodes a 261 residues protein kinase, piD261, now renamed Vsk1p (very small kinase 1), whose structurl homologues are present in a variety of organisms and whose function is unknown. We have previously demonstrated that the protein is a S/T kinase and here we show, by mutational analysis, that the invariant residues of protein kinase are all conserved in Vsk1p, but are embedded in a altered context, suggestive of unique mechanistic properties. The biological competence of the Vsk1p protein is correlated with its phosphotransferase activity, as the phenotype due to the VSK1 gene disruption is not, or partially, complemented by ectopic expression of Vsk1p point mutants which are catalitically inactive in vitro. However, the absence of the protein causes in the cell more dramatic effects thn its presence in a catalitically inactive form and this is possibly due to the failure to form protein complex(es) within the cell. In order to understand the actual role of the protein, we have started a search for protein(s) functionally associated with Vsk1p by the 2-hybrid approach. Two of the interacting proteins could be good candidates to correlate the function of this protein with the manifested phenotypes.