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Molecular and Cellular Biology, January 2001, p. 488-500, Vol. 21, No. 2
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.2.488-500.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

The Phosphotyrosyl Phosphatase Activator, Ncs1p (Rrd1p), Functions with Cla4p To Regulate the G2/M Transition in Saccharomyces cerevisiae

David A. Mitchell and George F. Sprague Jr.*

Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403-1229

Received 3 July 2000/Returned for modification 15 August 2000/Accepted 13 October 2000

The Saccharomyces cerevisiae p21-activated kinases, Ste20p and Cla4p, have individual functions but appear to share an essential function(s) as well because a strain lacking both kinases is inviable. To learn more about the shared function, we sought new mutations that were lethal in the absence of CLA4. This approach led to the identification of at least 10 complementation groups designated NCS (need CLA4 to survive). As for ste20 cla4-75 mutants, most ncs cla4-75 double mutants were defective for septin localization during budding. One group, NCS1/RRD1 (YIL153w), did not confer this defect, however, and we investigated its function further. ncs1Delta cla4Delta cells arrested with elongated buds and short mitotic spindles. The morphological defects and lethality were suppressed by mutations that abrogate the cell cycle morphogenetic checkpoint, CDC28Y19F or swe1Delta . The connection to the cell cycle may be direct, as we detected a Cla4p-Cdc28p complex. NCS1 encodes a protein with significant similarity to a mammalian phosphotyrosyl phosphatase activator (PTPA) regulatory subunit for type 2A protein phosphatases (PP2As). Genetic and biochemical evidence suggested that the phosphatase Sit4p is a target for Ncs1p. First, CLA4 and SIT4 were synthetically lethal. Second, Ncs1p and its yeast paralog, Noh1p (Rrd2p), bound to the catalytic domain of Sit4p in vitro, and Ncs1p could be immunoprecipitated with Sit4p but not with another PP2A (Pph21p) from yeast cell extracts. Strains lacking both NCS1 and NOH1 were inviable and arrested as unbudded cells, implying that PTPA function is required for proper G1 progression.

* Corresponding author. Mailing address: Institute of Molecular Biology, University of Oregon, Eugene, OR 97403-1229. Phone: (541) 346-5158. Fax: (541) 346-5891. E-mail: gsprague{at}molbio.uoregon.edu.

Molecular and Cellular Biology, January 2001, p. 488-500, Vol. 21, No. 2
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.2.488-500.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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