Role of Sds22 in the regulation of protein phosphatase-1 in Saccharomyces cerevisiae
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] GLC7 encodes the catalytic subunit of protein phosphatase 1 (PP1) in the budding yeast Saccharomyces cerevisiae. Glc7 binds different regulatory subunits which specifically localize and also specify the substrates of the catalytic subunit. Sds22 is an essential regulator of Glc7. Sds22 binds and localizes Glc7 in the nucleus. The substrates of Glc7-Sds22 are thought to be the kinetochore proteins. The mode of regulation of Glc7 by Sds22 is not exactly known. The objective of this research was to further elucidate regulation of PP1 by Sds22. We hypothesized that in addition to binding and localizing Glc7 in the nucleus, Sds22 binds the proteins/ substrates essential for the Glc7-Sds22 function. First, we characterized STS, a potential bypass suppressor of Sds22. The data suggest that STS might be a duplication of chromosome XI that has SDS22. The duplication of chromosome could have resulted from overexpression of Glc7 in sds22 strain. Second, we confirmed binding of seven different GST-fusion proteins (Kog1, Nop6, Net, Rvb1, Rvb2, Snf4 and YGR130C) to Sds22 by co-immunoprecipitation assay. We discovered that Bub3, a mitotic spindle assembly checkpoint protein, also binds Sds22, which was not known previously. In order to determine whether the binding of Glc7 is required for Sds22 to suppress ipl1 temperature sensitivity, we made different missense and deletion mutations in the leucine rich repeats (LRRs) of SDS22. The stable missense and LRR deletion Sds22 mutant proteins continued to bind Glc7, though with reduced efficiency compared to the wild-type Sds22 protein. The mutant Sds22 proteins that have reduced affinity for Glc7 suppressed ipl1 with reduced efficiency. Collectively, our results suggest that Sds22 has more functions than just binding and localizing Glc7 in the nucleus. Sds22 binds proteins in addition to binding Glc7 and requires both the binding of Glc7 and essential proteins/ substrates for the full or best suppression of ipl1-1.
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