Identification and characterization of the CLK1 gene product, a novel CaM kinase-like protein kinase from the yeast Saccharomyces cerevisiae

The CLK1 gene of Saccharomyces cerevisiae encodes a 610-residue protein kinase that resembles known type II Ca2+/calmodulin-dependent protein kinases (CaM kinases), including the CMK1 and CMK2 gene products from the same yeast, The Clk1 kinase domain is preceded by a 162-residue N-terminal extension, followed by a 132-residue C-terminal extension (which contains a basic segment resembling known calmodulin-binding sites) and is as similar to mammalian CaM kinase (38% identity to rat CaM kinase cu) as it is to yeast CaM kinase (37% identity to Cmk2). However, Clk1 shares 52% identity with Rck1, another putative protein kinase encoded in the S. cerevisiae genome, Clk1 tagged with a c-myc epitope (expressed in yeast) and a GST-Clk1 fusion (expressed in bacteria) underwent autophosphorylation and phosphorylated an exogenous substrate (yeast protein synthesis elongation factor 2), primarily on Ser. Neither Clk1 activity was stimulated by purified yeast calmodulin (CMD1 gene product), with or without Ca2+; no association of Clk1 with Cmd1 was detectable by other methods, C-terminally truncated Clk1(Delta 487-610) was growth-inhibitory when overexpressed, whereas catalytically inactive Clk1(K201R Delta 487-610) was not, suggesting that the C terminus is a negative regulatory domain. Using immunofluorescence, Clk1 was localized to the cytosol and excluded from the nucleus, A clk1 Delta. mutant, a clk1 Delta rck1 Delta double mutant, a clk1 Delta cmk1 Delta cmk2 Delta triple mutant, and a clk1 Delta rck1 Delta cmk1 Delta cmk2 Delta quadruple mutant were all viable and manifested no other overt growth phenotype.