Deciphering the Role of p24 Proteins in COPII-mediated Protein Secretion

In eukaryotic cells, proteins continuously flux through the organelles of the secretory pathway in an essential cellular process called protein secretion. This dynamic process originates at the endoplasmic reticulum (ER), where translating ribosomes push linear peptides into the ER membrane and lumen. ER chaperones assist in folding nascent peptides into three-dimensional conformations and proteins are concentrated into membrane-encapsulated vesicles bound for the Golgi apparatus. ER to Golgi transport is mediated by a set of cytosolic coat proteins called COPII. The COPII coat polymerizes into a lattice on the ER membrane that is able to bend the membrane around secretory cargo and bud off a spherical vesicle. Protein secretion is subject to rapid changes as a cell responds to its environment and requirements for viability alter. In addition to accommodating short-term demands, such as translational up-regulation, evolved complexity of secretory proteins over time, has also required that secretory components adapt. In both cases changes in secretory demands require that the COPII proteins have an inherent flexibility to navigate these changes without disrupting secretory flux. In this work I have examined a family of quintessential secretory cargo, p24 proteins, that challenge protein secretion. This family of proteins forms a hetero-tetrameric complex that cycles between the ER and the Golgi and mediates transport of glycosylphosphatidylinositol-anchored proteins (GPI-APs). Here I present evidence that suggests, when present in vesicles, both p24 proteins and their GPI-AP cargo present a challenge to vesicle formation. I posit that three attributes of these proteins present a local barrier to membrane bending: Lumenal asymmetric distribution across the membrane, high cellular abundance and affinity for ceramide rich membranes. I have also elucidated mechanisms that the coat has evolved to accommodate troublesome cargo such as p24 proteins, which enhance structural scaffolding and increase average vesicle size. Finally I present preliminary findings indicating that p24s also contribute to ER homeostasis by preventing aberrant incorporation of proteins into vesicles. Comprehensively, these findings have shed light on the role of p24 proteins in vesicles. Traditionally thought to be canonical ER cargo receptors, these proteins also appear capable of contributing to the composition of the vesicles in which they reside, and impacting trafficking efficiency in two ways: First by directly mediating transport of GPI-APs and second by uniformly packing vesicles to avoid wasteful secretion. My work has contributed to a growing notion in the field that secretory cargo are not inert passengers but active participants in vesicle mediated secretion.