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Unconventional secretion of Acb1 is mediated by autophagosomes
Authors:Juan M. Duran  Christophe Anjard  Chris Stefan  William F. Loomis  Vivek Malhotra
Affiliation:1.Center for Genomic Regulation, Barcelona 08003, Spain;2.Department of Cell and Developmental Biology, University of California, San Diego, La Jolla, CA 92093;3.Weill Institute for Cell and Molecular Biology, Cornell University, Ithaca, NY 14850;4.Institutó Catalana de Recerca i Estudis Avançats, Barcelona 08003, Spain
Abstract:Starving Dictyosteliumdiscoideum cells secrete AcbA, an acyl coenzyme A–binding protein (ACBP) that lacks a conventional signal sequence for entering the endoplasmic reticulum (ER). Secretion of AcbA in D. discoideum requires the Golgi-associated protein GRASP. In this study, we report that starvation-induced secretion of Acb1, the Saccharomyces cerevisiae ACBP orthologue, also requires GRASP (Grh1). This highlights the conserved function of GRASP in unconventional secretion. Although genes required for ER to Golgi or Golgi to cell surface transport are not required for Acb1 secretion in yeast, this process involves autophagy genes and the plasma membrane t-SNARE, Sso1. Inhibiting transport to vacuoles does not affect Acb1 secretion. In sum, our experiments reveal a unique secretory pathway where autophagosomes containing Acb1 evade fusion with the vacuole to prevent cargo degradation. We propose that these autophagosome intermediates fuse with recycling endosomes instead to form multivesicular body carriers that then fuse with the plasma membrane to release cargo.
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