VAMP-associated protein-A regulates partitioning of oxysterol-binding protein-related protein-9 between the endoplasmic reticulum and Golgi apparatus

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We recently showed that oxysterol-binding protein (OSBP), one of twelve related PH domain containing proteins with lipid and sterol binding activity, interacts with VAMP-associated protein (VAP)-A on the endoplasmic reticulum (ER). In addition to OSBP, seven OSBP-related proteins (ORPs) bind VAP-A via a conserved E-F/Y-F/Y-DA ‘FFAT’ motif. We focused on this interaction for ORP9, which is expressed as a full-length (ORP9L) or truncated version missing the PH domain (ORP9S). Mutation analysis showed that the interaction required the ORP9 FFAT motif and the N-terminal conserved domain of VAP. Endogenous ORP9L displayed Golgi localization, which was partially mediated by the PH domain based on limited localization of OPR9–PH–GFP with the Golgi apparatus. When inducibly overexpressed, ORP9S and ORP9L colocalized with VAP-A and caused vacuolation of the ER as well as retention of the ER–Golgi intermediate compartment marker ERGIC-53/p58 in the ER. ORP9L mutated in the VAP-A binding domain (ORP9L-FY→AA) did not localize to the ER but appeared with giantin and Sec31 on large vesicular structures, suggesting the presence of a hybrid Golgi–COPII compartment. Normal Golgi localization was also observed for ORP9L-FY→AA. Results show that VAP binding and PH domains target ORP9 to the ER and a Golgi–COPII compartment, respectively, and that ORP9L overexpression in these compartments severely perturbed their organization.

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