γ-BAR, a novel AP-1-interacting protein involved in post-Golgi trafficking

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Abstract
A novel peripheral membrane protein (2c18) that interacts directly with the gamma ‘ear’ domain of the adaptor protein complex 1 (AP‐1) in vitro and in vivo is described. Ultrastructural analysis demonstrates a colocalization of 2c18 and γ1‐adaptin at the trans ‐Golgi network (TGN) and on vesicular profiles. Overexpression of 2c18 increases the fraction of membrane‐bound γ1‐adaptin and inhibits its release from membranes in response to brefeldin A. Knockdown of 2c18 reduces the steady‐state levels of γ1‐adaptin on membranes. Overexpression or downregulation of 2c18 leads to an increased secretion of the lysosomal hydrolase cathepsin D, which is sorted by the mannose‐6‐phosphate receptor at the TGN, which itself involves AP‐1 function for trafficking between the TGN and endosomes. This suggests that the direct interaction of 2c18 and γ1‐adaptin is crucial for membrane association and thus the function of the AP‐1 complex in living cells. We propose to name this protein γ‐BAR.