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Published online 11 August 2003. doi:10.1083/jcb.200302038
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© The Rockefeller University Press, 0021-9525/2003/8/635 $5.00
The Journal of Cell Biology, Volume 162, Number 4, 635-646

Article

 The GAP activity of Msb3p and Msb4p for the Rab GTPase Sec4p is required for efficient exocytosis and actin organization

Xiang-Dong Gao1, Stefan Albert2, Serguei E. Tcheperegine1, Christopher G. Burd1, Dieter Gallwitz2 and Erfei Bi1

1 Department of Cell and Developmental Biology, University of Pennsylvania, Philadelphia, PA 19104
2 Max Planck Institute for Biophysical Chemistry, Department of Molecular Genetics, D-37070 Göttingen, Germany

Address correspondence to Erfei Bi, Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Room 1012, BRB II/III, 421 Curie Blvd., Philadelphia, PA 19104-6058. Tel.: (215) 573-6676. Fax: (215) 898-9871. email: ebi{at}mail.med.upenn.edu

Polarized growth in Saccharomyces cerevisiae is thought to occur by the transport of post-Golgi vesicles along actin cables to the daughter cell, and the subsequent fusion of the vesicles with the plasma membrane. Previously, we have shown that Msb3p and Msb4p genetically interact with Cdc42p and display a GTPase-activating protein (GAP) activity toward a number of Rab GTPases in vitro. We show here that Msb3p and Msb4p regulate exocytosis by functioning as GAPs for Sec4p in vivo. Cells lacking the GAP activity of Msb3p and Msb4p displayed secretory defects, including the accumulation of vesicles of 80–100 nm in diameter. Interestingly, the GAP activity of Msb3p and Msb4p was also required for efficient polarization of the actin patches and for the suppression of the actin-organization defects in cdc42 mutants. Using a strain defective in polarized secretion and actin-patch organization, we showed that a change in actin-patch organization could be a consequence of the fusion of mistargeted vesicles with the plasma membrane.

Key Words: actin; exocytosis; Msb3p; Msb4p; Rab GAP


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