Spy1, a histidine-containing phosphotransfer signaling protein, regulates the fission yeast cell cycle through the Mcs4 response regulator

Common histidine-to-aspartate (His-to-Asp) phosphorelay signaling systems i nvolve three types of signaling components: a sensor His kinase, a response regulator, and a histidine-containing phosphotransfer (HPt) protein. In th e fission yeast Schizosaccharomyces pombe, two response regulators, Mcs4 an d Prr1, have been identified recently, and it was shown that they are invol ved in the signal transduction implicated in stress responses. Furthermore, Mcs4 appears to be involved in mitotic cell-cycle control. However, neithe r the HPt phosphotransmitter nor His kinase has been characterized in S. po mbe. In this study, we identified a gene encoding an HPt phosphotransmitter , named Spy1 (S. pombe YPD1-like protein). The spy1(+) gene showed an abili ty to complement a mutational lesion of the Saccharomyces cerevisiae YPD1 g ene, which Is involved in an osmosensing signal transduction. The result fr om yeast two-hybrid analysis indicated that Spy1 interacts with Mcs4. To ga in insight into the function of Spy1, a series of genetic analyses were con ducted. The results provided evidence that Spy1, together with Mcs4 plays a role in regulation of the G(2)/M cell cycle progression. Spy1-deficient ce lls appear to be precocious in the entry to M phase. In the proposed model, Spy1 modulates Mcs4 in a negative manner, presumably through a direct His- to-Asp phosphorelay, operating upstream of the Sty1 mitogen-activated prote in kinase cascade.