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© The Rockefeller University Press, 0021-9525/1997//1495 $5.00
The Journal of Cell Biology, Volume 137, Number 7, , 1997 1495-1509

Article

Sec2p Mediates Nucleotide Exchange on Sec4p and Is Involved in Polarized Delivery of Post-Golgi Vesicles



Christiane Walch-Solimena, Ruth N. Collins, and Peter J. Novick

Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06510

The small GTPase Sec4p is required for vesicular transport at the post-Golgi stage of yeast secretion. Here we present evidence that mutations in SEC2, itself an essential gene that acts at the same stage of the secretory pathway, cause Sec4p to mislocalize as a result of a random rather than a polarized accumulation of vesicles. Sec2p and Sec4p interact directly, with the nucleotide-free conformation of Sec4p being the preferred state for interaction with Sec2p. Sec2p functions as an exchange protein, catalyzing the dissociation of GDP from Sec4 and promoting the binding of GTP. We propose that Sec2p functions to couple the activation of Sec4p to the polarized delivery of vesicles to the site of exocytosis.

1. Abbreviations used in this paper: EST, expressed sequence tag; GEF, guanine nucleotide exchange factor; MBP, maltose binding protein; t-SNARE, target SNAP receptor; v-SNARE, vesicle SNAP receptor.

We are deeply indebted to Dr. B. Govindan for help with electron microscopy experiments, her interest in the project, and helpful discussions. We thank Dr. K.A. Joiner and S. Kupfer for providing constructs, L. Chicone and L. Hartnell for help and advice in electron microscopy techniques, Dr. C.W. Slayman for the plasma membrane ATPase antibody, Dr. G. Whittaker for experimental advice, and Dr. M. Solimena for critical reading of the manuscript.

Christiane Walch-Solimena and Ruth Collins contributed equally to this paper.

Please address all correspondence to Peter J. Novick, Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510. Tel.: (203) 785-5871; Fax: (203) 785-7226; E-mail: peter_novick{at}quickmail.cis.yale.edu


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