Structural and Functional Characterization of Membrane Budding and Fusion Proteins at the ER

Abstract

Eukaryotic cells utilize vesicles to transport proteins and lipids between cellular compartments. COPII-coated vesicles are generated at the endoplasmic reticulum (ER). Their formation is initiated when a guanine nucleotide exchange factor (GEF) protein, Sec12, activates the GTPase Sar1. Once activated, Sar1 binds to the ER membrane and recruits COPII coat proteins. These COPII coat components act in concert to sculpt a vesicle from the membrane. We have determined the structure of Sec12, finding that it is a beta-propeller with an evolutionarily conserved patch of residues responsible for its interaction with Sar1. We observe that a potassium binding loop, the "K loop", lies within this conserved region and demonstrate that it is essential for GEF activity in vitro and viability in yeast. Furthermore, potassium, by binding to the K loop, markedly increases GEF activity.

At the completion of their journey, vesicles are tethered to and then fuse with their target membrane, delivering their cargo. The Dsl1 tethering complex is thought to mediate these final vesicle transport steps at the ER membrane by tethering incoming COPI-coated vesicles and facilitating SNARE complex formation. We report a novel role for the Dsl1 complex as a mediator of ER homotypic fusion, a process necessary for maintaining the complex architecture of the ER. This ER fusion pathway is redundant with another, Sey1-mediated, fusion pathway; involves the Dsl1 complex and the SNAREs Use1, Sec20, and Ufe1; and is coatomer independent. Thus, the Dsl1 complex appears to have two different functional roles, one in vesicle docking/fusion and the second in homotypic ER fusion.