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Vol. 16, Issue 10, 4543-4556, October 2005

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Molecular Interactions Position Mso1p, a Novel PTB Domain Homologue, in the Interface of the Exocyst Complex and the Exocytic SNARE Machinery in Yeast

Michael Knop ^*, K. Juha Miller , Massimiliano Mazza ^*, DeJiang Feng  , Marion Weber , Sirkka Keränen , and Jussi Jäntti  

^* EMBL, Heidelberg D-69117, Germany; VTT Biotechnology, FIN-02044 VTT Espoo, Finland; and Institute of Biotechnology, University of Helsinki, 00014 University of Helsinki, Finland

Submitted March 22, 2005; Revised May 25, 2005; Accepted July 7, 2005
Monitoring Editor: Jennifer Lippincott-Schwartz

In this study, we have analyzed the association of the Sec1p interacting protein Mso1p with the membrane fusion machinery in yeast. We show that Mso1p is essential for vesicle fusion during prospore membrane formation. Green fluorescent protein-tagged Mso1p localizes to the sites of exocytosis and at the site of prospore membrane formation. In vivo and in vitro experiments identified a short amino-terminal sequence in Mso1p that mediates its interaction with Sec1p and is needed for vesicle fusion. A point mutation, T47A, within the Sec1p-binding domain abolishes Mso1p functionality in vivo, and mso1T47A mutant cells display specific genetic interactions with sec1 mutants. Mso1p coimmunoprecipitates with Sec1p, Sso1/2p, Snc1/2p, Sec9p, and the exocyst complex subunit Sec15p. In sec4-8 and SEC4I133 mutant cells, association of Mso1p with Sso1/2p, Snc1/2p, and Sec9p is affected, whereas interaction with Sec1p persists. Furthermore, in SEC4I133 cells the dominant negative Sec4I133p coimmunoprecipitates with Mso1p–Sec1p complex. Finally, we identify Mso1p as a homologue of the PTB binding domain of the mammalian Sec1p binding Mint proteins. These results position Mso1p in the interface of the exocyst complex, Sec4p, and the SNARE machinery, and reveal a novel layer of molecular conservation in the exocytosis machinery.

Address correspondence to: Jussi Jäntti (jussi.jantti{at}helsinki.fi) or Michael Knop (knop{at}embl.de).

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