Exocytosis is a eukaryotic process in which vesicles deliver membrane and other cargoes to and across the plasma membrane. The exocyst is a tethering complex necessary for the polarization and fusion of exocytic vesicles with the plasma membrane. It is conserved in eukaryotes, although it exhibits increasingly complex characteristics from yeasts to mammals. Exo70 is one of eight protein subunits of the exocyst. Its interactions with a Rho family GTPase and the Arp2/3 actin branching complex are important for exocytosis.The aim of this work is to gain greater insight into the structure and function of the exocyst and the role of Exo70 within it. The high-resolution structures of the C-terminal 90% of Exo70 from the yeast Saccharomyces cerevisiae and the mouse Mus musculus are presented here as determined by X-ray crystallography. These structures provide a unique opportunity to study a near-complete component of the exocyst and to compare and contrast this molecule between two distantly related model organisms. A conserved architecture composed of a series of unique helix-turn-helix motifs organized into a rod shape is revealed in these molecules despite low primary sequence conservation. A poor understanding of the role of this domain structure makes functional conclusions drawn from these structures difficult. These molecules also contain a novel fold that has recently been observed in other proteins participating in exocytosis. Several significant structural deviations between these molecules raise new questions about thefunction of Exo70 and the interactions in which it is involved. These structures may provide information important to future studies of the exocyst and the GTPases that interact with Exo70 in both budding yeast and mammals.
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Structures of Exocyst Subunit Exo70 from Yeast and Mouse.
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