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Vol. 16, Issue 10, 4595-4608, October 2005

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Cell Polarity Protein Spa2P Associates With Proteins Involved In Actin Function In Saccharomyces Cerevisiae

Judy L. Shih ^* , Samara L. Reck-Peterson   ^||, Rick Newitt ^¶, Mark S. Mooseker   ^#, Ruedi Aebersold ^¶ @, and Ira Herskowitz ^* **

^* Department of Biochemistry and Biophysics, University of California–San Francisco, San Francisco, CA 94143-2140; Department of Molecular, Cellular, and Developmental Biology, New Haven, CT 06520-8103; Department of Cell Biology, New Haven, CT 06520-8103; ^# Department of Pathology, Yale University, New Haven, CT 06520-8103; and ^¶ Institute for Systems Biology, Seattle, WA 98103-8904

Submitted February 9, 2005; Revised June 10, 2005; Accepted June 13, 2005
Monitoring Editor: David Drubin

Spa2p is a nonessential protein that regulates yeast cell polarity. It localizes early to the presumptive bud site and remains at sites of growth throughout the cell cycle. To understand how Spa2p localization is regulated and to gain insight into its molecular function in cell polarity, we used a coimmunoprecipitation strategy followed by tandem mass spectrometry analysis to identify proteins that associate with Spa2p in vivo. We identified Myo1p, Myo2p, Pan1p, and the protein encoded by YFR016c as proteins that interact with Spa2p. Strikingly, all of these proteins are involved in cell polarity and/or actin function. Here we focus on the functional significance of the interactions of Spa2p with Myo2p and Myo1p. We find that localization of Spa2GFP to sites of polarized growth depends on functional Myo2p but not on Myo1p. We also find that Spa2p, like Myo2p, cosediments with F-actin in an ATP-sensitive manner. We hypothesize that Spa2p associates with actin via a direct or indirect interaction with Myo2p and that Spa2p may be involved in mediating polarized localization of polarity proteins via Myo2p. In addition, we observe an enhanced cell-separation defect in a myo1spa2 strain at 37°C. This provides further evidence that Spa2p is involved in cytokinesis and cell wall morphogenesis.

Present addresses: Department of Medicine, Beth Israel Deaconess Medical Center, Harvard University, Boston, MA 02215;

^|| Present addresses: Department of Cellular and Molecular Pharmacology, University of California–San Francisco, San Francisco, CA 94143-2200;

^@ Present addresses: Institute for Molecular Systems Biology, ETH Hönggerberg HPT E 78, Wolfgang Pauli-Straße 16, CH-8093 Zürich, Switzerland.

^** Deceased April 28, 2003.

Address correspondence to: Judy Shih (jshih{at}bidmc.harvard.edu).

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