Every time a cell secretes a protein, it must first package that protein into a bubble-like carrier that buds off a membrane—but the machinery that builds these carriers must somehow adapt to handle cargoes ranging from tiny enzymes to massive collagen fibres 300 nanometres long. Current understanding of the COPII coat, the protein complex that sculpts these carriers, explains how it forms standard 60–80 nanometre vesicles but not how it generates the larger, differently shaped carriers needed for collagen or cholesterol-laden lipoproteins. This gap matters because abnormal collagen secretion drives tissue fibrosis, and excessive cholesterol secretion fuels heart disease—both major health burdens with few treatment options. The team proposes that two floppy, disordered regions within COPII proteins—one controlling the coat’s assembly timing, the other its membrane-bending force—are the key to carrier diversity. Using cryo-electron microscopy, stem cells, and real-time imaging, they will test how these domains tune carrier size and shape. This is fundamental science. Success would reveal a dynamic mechanism that cells use to meet specific secretion needs. Beyond this pathway, the principles could inform other membrane trafficking systems that handle diverse cargoes. A deeper mechanistic understanding could eventually guide development of small molecules that reprogramme carriers, offering a new way to manipulate secretion therapeutically.
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One third of the human genome encodes proteins that enter the secretory pathway for delivery to internal compartments or release outside the cell. Secreted proteins come in many different shapes and sizes, and they perform diverse functions important to human health. For example, collagen is a long (300nm) fibre that forms part of the extracellular matrix that binds cells into tissues. Abnormal collagen secretion causes tissue fibrosis, for which there are few treatments. Cholesterol-containing lipoprotein particles are large structures secreted into the bloodstream to deliver this essential molecule throughout the body. Excessive cholesterol secretion drives heart disease, a major cause of mortality. This research programme aims to understand how diverse secreted proteins are efficiently exported from their site of synthesis to initiate their secretion. By understanding the mechanisms of the first step in secretion, we hope to develop new ways to selectively manipulate this pathway, thereby potentially impacting a spectrum of diseases. Despite their varied structures and functions, all secreted proteins share a common biogenesis pathway. After their synthesis in the endoplasmic reticulum (ER), secreted proteins are packaged into membrane-bound transport carriers that ferry them to the next compartment in the secretory pathway. Coat protein type II (COPII) is a conserved set of proteins that self-assemble into a “coat” on the ER membrane to simultaneously bind secretory cargo proteins and sculpt transport carriers from the ER membrane. Although the basic mechanism of COPII function has long been established, we currently lack insight into how these proteins generate carriers of different size and shapes capable of transporting diverse secreted proteins. Our central hypothesis is that COPII assembly into a macromolecular complex capable of generating transport carriers is a dynamic process that can be tuned to meet a cell’s specific needs. Fibroblasts and hepatocytes must generate carriers large enough to encapsulate collagen and lipoprotein particles respectively. Other secretory cells generate canonical 60-80nm transport intermediates that handle small secreted proteins. We are an interdisciplinary team using cutting-edge technology to dissect the molecular basis of COPII coat assembly in the context of specific cellular physiology. We propose that intrinsically disordered regions (IDRs) of two COPII proteins, Sec31 and Sec24, play central roles in generation of diverse carriers. The Sec31 IDR likely controls the timing of coat assembly/disassembly cycles, and we will test the mechanisms by which this domain influences the lifetime of the coat on membranes, and how this timing mechanism in turn influences carrier morphology. The Sec24 IDR directly influences carrier morphology, probably by controlling the membrane-bending force that sculpts carriers. We will dissect the biophysical mechanisms by which this occurs, and how specific IDR properties yield different membrane curvatures. Our multidisciplinary approach leverages team expertise in genetics and biochemistry to dissect mechanism, cryo-electron microscopy to visualize carriers within cells, human stem cells to explore physiology, and real-time imaging to reveal system dynamics. At the conclusion of this programme, we will have gained fundamental insight into a dynamic biological system that has essential functions in human health. Our understanding of the COPII coat system will inform related conceptual problems in other membrane trafficking pathways that similarly handle diverse sized cargoes. Finally, our mechanistic dissection of specific protein domains will yield a pipeline for small molecule development of modulators that can reprogramme carriers to manipulate secretion for therapeutic advantage.
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