Vesicles transport

In addition to secretory cells, many non-secretory cells are capable of regulating exocytotic fusion of transport vesicles that are derived from endosomal precursors. For instance, vesicles enriched in plasma membrane transport proteins are incorporated in a regulated manner in order to alter metabolite fluxes. Examples include the glucose transporter GLUT-4 in muscle and fat tissues, a key element in the control of... 

Along their route through the Golgi, secretory and membrane proteins destined for the various post-Golgi pathways are intermixed. Thus, proteins of distinct routes, i.e. the endosomal and the secretory route, are sorted into individual types of transport vesicles at the TGN. Among the best characterized types of TGN-derived vesicles are clathrin-coated vesicles. In addition, several types of non-clathrin-coated vesicles have been identified but their specific functions remain to be characterized. 

Rothman JE, Wieland FT (1996) Protein sorting by transport vesicles. Science 272 227-234... 

Microtubule-associated proteins bind to microtubules in vivo and subserve a number of functions including the promotion of microtubule assembly and bundling, chemomechanical force generation, and the attachment of microtubules to transport vesicles and organelles (Olmsted, 1986). Tubulin purified from brain tissue by repeated polymerization-depolymerization contains up to 20% MAPs. The latter can be dissociated from tubulin by ion-exchange chromatography. The MAPs from brain can be resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). 

Lasek, R.J. Brady, S.T. (1985). Attachment of transported vesicles to micrombules in axoplasm is facilitated by AMP-PNP. Nature 316, 645-647. 

This sequence specifies that such proteins will be attached to the inner aspect of the ER in a relatively loose manner. The chaperone BiP (see below) is one such protein. Actually, KDEL-containing proteins first travel to the Golgi, interact there with a specific KDEL receptor protein, and then return in transport vesicles to the ER, where they dissociate from the receptor. 

TRANSPORT VESICLES ARE KEY PLAYERS IN INTRACELLULAR PROTEIN TRAFFIC... 

Most proteins that are synthesized on membrane-bound polyribosomes and are destined for the Golgi apparatus or plasma membrane reach these sites inside transport vesicles. The precise mechanisms by which proteins synthesized in the rough ER are inserted into these vesicles are not known. Those involved in transport from the ER to the Golgi apparatus and vice versa—and from the Golgi to the plasma membrane— are mainly clathrin-free, unlike the coated vesicles involved in endocytosis (see discussions of the LDL receptor in Chapters 25 and 26). For the sake of clarity, the non-clathrin-coated vesicles will be referred to in... 

Coatomer A family of at least seven coat proteins (a, 3, y, 8, , P, and Q. Different transport vesicles have different complements of coat proteins. 

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. 

Membrane proteins inside transport vesicles bud off the endoplasmic reticulum on their way to the Golgi final sorting of many membrane proteins occurs in the trans-Golgi network. 

Many proteins synthesized on membrane-bound polyribosomes proceed to the Golgi apparatus and the plasma membrane in transport vesicles. 

Most transport vesicles bud off as coated vesicles, with a unique set of proteins decorating their cytosolic surface 141 GTP-binding proteins, such as the small monomeric GTPases and heterotrimeric GTPases (G proteins) facilitate membrane transport 142 SNARE proteins and Rabs control recognition of specific target membranes 143... 

Unloading of the transport vesicle cargo to the target membrane occurs by membrane fusion 143... 

Neurons constitute the most striking example of membrane polarization. A single neuron typically maintains thousands of discrete, functional microdomains, each with a distinctive protein complement, location and lifetime. Synaptic terminals are highly specialized for the vesicle cycling that underlies neurotransmitter release and neurotrophin uptake. The intracellular trafficking of a specialized type of transport vesicles in the presynaptic terminal, known as synaptic vesicles, underlies the ability of neurons to receive, process and transmit information. The axonal plasma membrane is specialized for transmission of the action potential, whereas the plasma... 

Another potential mechanism for protein retention is that ER proteins bind to each other to form large complexes that cannot be recruited in transport vesicles [27], This mechanism is known as kin recognition. Because the concentration of resident ER proteins can be very high (some had been estimated in the millimolar range), it can be achieved even with low-affinity interactions among resident proteins. 

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