Abstract
BackgroundCalcium/calmodulin-dependent protein kinase kinase (CaMKK) is required for diverse cellular functions. Mammalian CaMKK activates CaMKs and also the evolutionarily-conserved AMP-activated protein kinase (AMPK). The fission yeast Schizosaccharomyces pombe CaMKK, Ssp1, is required for tolerance to limited glucose through the AMPK, Ssp2, and for the integration of cell growth and division through the SAD kinase Cdr2.ResultsHere we report that Ssp1 controls the G2/M transition by regulating the activity of the CaMK Srk1. We show that inhibition of Cdc25 by Srk1 is regulated by Ssp1; and also that restoring growth polarity and actin localization of ssp1-deleted cells by removing the actin-monomer-binding protein, twinfilin, is sufficient to suppress the ssp1 phenotype.ConclusionsThese findings demonstrate that entry into mitosis is mediated by a network of proteins, including the Ssp1 and Srk1 kinases. Ssp1 connects the network of components that ensures proper polarity and cell size with the network of proteins that regulates Cdk1-cyclin B activity, in which Srk1 plays an inhibitory role.
Highlights
Among the Ca2+/CaM-regulated enzymes found in eukaryotic cells, the multifunctional Ca2+/ calmodulin-dependent protein kinases (CaMKs) occupy positions of influence because they communicate the Ca2+ signal via phosphorylation to a wide range of substrates [1,2]
We show that inhibition of Cdc25 by Srk1 is regulated by Ssp1; and that restoring growth polarity and actin localization of ssp1-deleted cells by removing the actin-monomerbinding protein, twinfilin, is sufficient to suppress the ssp1 phenotype
These findings demonstrate that entry into mitosis is mediated by a network of proteins, including the Ssp1 and Srk1 kinases
Summary
Among the Ca2+/CaM-regulated enzymes found in eukaryotic cells, the multifunctional Ca2+/ calmodulin-dependent protein kinases (CaMKs) occupy positions of influence because they communicate the Ca2+ signal via phosphorylation to a wide range of substrates [1,2]. In the genome of S. pombe, five genes code for proteins that have a high similarity with mammalian CaMK sequences: Cmk, Cmk and Srk show sequence similarity to CaMKs and; while Ckk and Ssp shows sequence similarity with CaMKKs. Among these, only the activity of Cmk kinase has been proven to be Ca2+/CaM-dependent [6,7]. Ssp and Pom have recently been identified as part of a mechanism that controls cell growth and division, in which the SAD family kinase Cdr plays a key role [23]. The fission yeast Schizosaccharomyces pombe CaMKK, Ssp, is required for tolerance to limited glucose through the AMPK, Ssp, and for the integration of cell growth and division through the SAD kinase Cdr
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