Specificity and Functional Conservation of COPII Components

Abstract

Protein secretion into the extracellular space is a fundamental process in all eukaryotic cells. In multicellular organisms, secreted proteins participate in critical processes such as intercellular communication, maintenance of metabolic homeostasis, immune reactions, and neurotransmission. The majority of secreted proteins follow the intracellular secretory pathway starting from the endoplasmic reticulum (ER). The first step in this pathway is the recruitment and transport of these proteins from the ER to the Golgi apparatus by coat protein complex II (COPII). Cargo recruitment is mediated by COPII protein in conjunction with cargo receptors – ER transmembrane proteins that bridge interactions between COPII proteins and soluble proteins restricted to the ER lumen. This dissertation examines the specificity of this process. We explored whether the two mammalian paralogs for the COPII component SAR1 are functionally equivalent and found that they exhibit extensive functional overlap such that SAR1A can compensate for the SAR1B deficiency in mice. We next focused on the two prototypical mammalian cargo receptors LMAN1 and SURF4. We developed a novel mass spectrometry-based analysis approach to identify bona fide secreted proteins with high sensitivity. We then applied this technique to define the cargo repertoire for LMAN1 and SURF4, finding that SURF4 has a much more extensive cargo range than LMAN1 in hepatocytes. Finally, we investigated the physiologic roles of hepatic SURF4 in mice, revealing that SURF4 mediates the secretion of lipoproteins from hepatocytes, with mice lacking hepatic SURF4 exhibiting profound hypocholesterolemia. Together, these data demonstrate both functional conservation among COPII paralogs and specificity of cargo receptors in mammals.