Transport vesicles also occur in various sizes and shapes. Diverse COPII vesicles are required for the transport of large cargo molecules. Collagen, for example, is assembled in the ER as 300-nm-long, stiff procollagen rods that then are secreted from the cell where they are cleaved by proteases to collagen, which is embedded into the extracellular matrix (discussed in Chapter 19). Procollagen rods do not fit into the 60–80 nm COPII vesicles normally observed. To circumvent this problem, the procollagen cargo molecules bind to transmembrane packaging proteins in the ER, which control the assembly of the COPII coat components (Figure 13–15). These events drive the local assembly of much larger COPII vesicles that accommodate the oversized cargo. Human mutations in genes encoding such packaging proteins result in collagen defects with severe consequences, such as skeletal abnormalities and other developmental defects. Similar mechanisms must regulate the sizes of vesicles required to secrete other large macromolecular complexes, including the lipoprotein particles that transport lipids out of cells.
Figure 13–14 Formation of a COPII-coated vesicle. (a) Inactive, soluble sar1-gdp binds to a sar1-gef in the er membrane, causing the sar1 to release its gdp and bind gTp. a gTp-triggered conformational change in sar1 exposes an amphiphilic helix, which inserts into the cytoplasmic leaflet of the er membrane, initiating membrane bending (which is not shown). (b) gTp-bound sar1 binds to a complex of two copII adaptor coat proteins, called sec23 and sec24, which form the inner coat. sec24 has several different binding sites for the cytosolic tails of cargo receptors. The entire surface of the complex that attaches to the membrane is gently curved, matching the diameter of copII-coated vesicles.
(c) a complex of two additional copII coat proteins, called sec13 and sec 31, forms the outer shell of the coat. like clathrin, they can assemble on their own into symmetrical cages with appropriate dimensions to enclose a copII-coated vesicle. (d) Membrane-bound, active sar1-gTp recruits copII adaptor proteins to the membrane. They select certain transmembrane proteins and cause the membrane to deform. The adaptor proteins then recruit the outer coat proteins which help form a bud. a subsequent membrane fusion event pinches off the coated vesicle. other coated vesicles are thought to form in a similar way. (c, modified from s.M. stagg et al., nature 439:234–238, 2006. with permission from Macmillan publishers ltd.)
Many other vesicle budding events likewise involve variations of common mechanisms. When living cells are genetically engineered to express fluorescent membrane components, the endosomes and trans Golgi network are seen in a fluorescence microscope to continually send out long tubules. Coat proteins assemble onto the membrane tubules and help recruit specific cargo. The tubules then either regress or pinch off with the help of dynamin-like proteins to form transport vesicles of different sizes and shapes.
Tubules have a higher surface-to-volume ratio than the larger organelles from which they form. They are therefore relatively enriched in membrane proteins compared with soluble cargo proteins. As we discuss later, this property of tubules is an important feature for sorting proteins in endosomes.
rab proteins guide Transport vesicles to Their Target Membrane