Monitoring Editor: Benjamin Glick The budding and fission of vesicles during membrane trafficking requires many proteins including those that coat the vesicles, adaptor proteins that recruit components of the coat, and small GTPases that initiate vesicle formation. In addition, vesicle formation in vitro is promoted by the hydrolysis of acyl-CoA lipid esters. The mechanisms by which these lipid esters are directed to the appropriate membranes in vivo, and what precisely their roles are in vesicle biogenesis is not yet understood. Here we present the first report on MAA-1, a novel membrane-associated member of the acyl-CoA-binding protein family. We show that in C. elegans, MAA-1 localizes to intracellular membrane organelles in the secretory and endocytic pathway, and that mutations in
maa-1 reduce the rate of endosomal recycling. A lack of
maa-1 activity causes a change in endosomal morphology. Whereas in wild type, many endosomal organelles have long tubular protrusions, loss of MAA-1 activity results in loss of the tubular domains, suggesting the
maa-1 is required the generation or maintenance of these domains. Furthermore, we demonstrate that MAA-1 binds fatty acyl-CoA in vitro and that this ligand binding ability is important for its function in vivo. Our results are consistent with a role for MAA-1 in an acyl-CoA-dependent process during vesicle formation.