Model of Non ClathrinCoated Vesicles Involves SNAREs Other Factors

Vesicles lie at the heart of intracellular transport of many proteins. Recently, significant progress has been made in understanding the events involved in vesicle formation and transport. This has transpired because of the use of a number of approaches. These include establishment of cell-free systems with which to study vesicle formation. For instance, it is possible to observe, by electron microscopy, budding of vesicles from Golgi preparations incubated with cytosol and ATP. The development of genetic approaches for studying vesicles in yeast has also been crucial. The picture is complex, with its own nomenclature (Table 46-7), and involves a variety of cytosolic and membrane proteins, GTP, ATP, and accessory factors.

Based largely on a proposal by Rothman and colleagues, anterograde vesicular transport can be considered to occur in eight steps (Figure 46-7). The basic concept is that each transport vesicle bears a unique address marker consisting of one or more v-SNARE proteins, while each target membrane bears one or more complementary t-SNARE proteins with which the former interact specifically.

Step 1: Coat assembly is initiated when ARF is activated by binding GTP, which is exchanged for GDP. This leads to the association of GTP-bound ARF with its putative receptor (hatched in Figure 46-7) in the donor membrane.

Table 46-7. Factors involved in the formation of non-clathrin-coated vesicles and their transport.

• ARF: ADP-ribosylation factor, a GTPase

• Coatomer: A family of at least seven coat proteins (a, p, y, 8, £, P', and Z). Different transport vesicles have different complements of coat proteins.

• SNAP: Soluble NSF attachment factor

• SNARE: SNAP receptor

• GTP-y-S: A nonhydrolyzable analog of GTP, used to test the involvement of GTP

• NEM: N-Ethylmaleimide, a chemical that alkylates sulfhy-dryl groups

• NSF: NEM-sensitive factor, an ATPase

• Rab proteins: A family of ras-related proteins first observed in rat brain; they are GTPases and are active when GTP is found

• Sec1: A member of a family of proteins that attach to t-SNAREs and are displaced from them by Rab proteins, thereby allowing v-SNARE-t-SNARE interactions to occur.

Step 2: Membrane-associated ARF recruits the coat proteins that comprise the coatomer shell from the cytosol, forming a coated bud.

Step 3: The bud pinches off in a process involving acyl-CoA—and probably ATP—to complete the formation of the coated vesicle.

Step 4: Coat disassembly (involving dissociation of

ARF and coatomer shell) follows hydrolysis of bound GTP; uncoating is necessary for fusion to occur.

Step 5: Vesicle targeting is achieved via members of a family of integral proteins, termed v-SNAREs, that tag the vesicle during its budding. v-SNAREs pair with cognate t-SNAREs in the target membrane to dock the vesicle.

It is presumed that steps 4 and 5 are closely coupled and that step 4 may follow step 5, with ARF and the coatomer shell rapidly dissociating after docking.

Step 6: The general fusion machinery then assembles on the paired SNARE complex; it includes an ATPase (NSF; NEM-sensitive factor) and the SNAP (soluble NSF attachment factor) proteins. SNAPs bind to the SNARE (SNAP receptor) complex, enabling NSF to bind.

Step 7: Hydrolysis of ATP by NSF is essential for fusion, a process that can be inhibited by NEM (N-ethylmaleimide). Certain other proteins and calcium are also required.

Figure 46-7. Model of the steps in a round of anterograde vesicular transport. The cycle starts in the bottom left-hand side of the figure, where two molecules of ARF are represented as small ovals containing GDP. The steps in the cycle are described in the text. Most of the abbreviations used are explained in Table 46-7. The roles of Rab and Secl proteins (see text) in the overall process are not dealt with in this figure. (CGN, cis-Golgi network; BFA, Brefeldin A.) (Adapted from Rothman JE: Mechanisms of intracellular protein transport. Nature 1994;372:55.) (Courtesy of E Degen.)

Figure 46-7. Model of the steps in a round of anterograde vesicular transport. The cycle starts in the bottom left-hand side of the figure, where two molecules of ARF are represented as small ovals containing GDP. The steps in the cycle are described in the text. Most of the abbreviations used are explained in Table 46-7. The roles of Rab and Secl proteins (see text) in the overall process are not dealt with in this figure. (CGN, cis-Golgi network; BFA, Brefeldin A.) (Adapted from Rothman JE: Mechanisms of intracellular protein transport. Nature 1994;372:55.) (Courtesy of E Degen.)

Step 8: Retrograde transport occurs to restart the cycle. This last step may retrieve certain proteins or recycle v-SNAREs. Nocodazole, a microtubule-disrupting agent, inhibits this step.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment