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Department of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, MI 48109

Correspondence to Jesse C. Hay: jesse.hay{at}mso.umt.edu

Abstract
What is the first membrane fusion step in the secretory pathway? In mammals, transport vesicles coated with coat complex (COP) II deliver secretory cargo to vesicular tubular clusters (VTCs) that ferry cargo from endoplasmic reticulum exit sites to the Golgi stack. However, the precise origin of VTCs and the membrane fusion step(s) involved have remained experimentally intractable. Here, we document in vitro direct tethering and SNARE-dependent fusion of endoplasmic reticulum–derived COPII transport vesicles to form larger cargo containers. The assembly did not require detectable Golgi membranes, preexisting VTCs, or COPI function. Therefore, COPII vesicles appear to contain all of the machinery to initiate VTC biogenesis via homotypic fusion. However, COPI function enhanced VTC assembly, and early VTCs acquired specific Golgi components by heterotypic fusion with Golgi-derived COPI vesicles.

J.C. Hay's present address is Division of Biological Sciences, and COBRE Center for Structural and Functional Neuroscience, The University of Montana, Missoula, MT 59812.

Abbreviations used in this paper: COP, coat complex; endo, endoglycosidase; NRK, normal rat kidney; VSV-G: vesicular stomatitis virus glycoprotein; VTC, vesicular tubular cluster.

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