Vesicle Traffic in the Early Stages of the Secretory Pathway
■ COPII vesicles transport proteins from the rough ER to the cis-Golgi; COPI vesicles transport proteins in the reverse direction (see Figure 17-14).
■ COPII coats comprise three components: the small GTP-binding protein Sar1, a Sec23/Sec24 complex, and a Sec13/Sec31 complex.
■ Components of the COPII coat bind to membrane cargo proteins containing a di-acidic or other sorting signal in their cytosolic regions (see Figure 17-15). Soluble cargo proteins probably are targeted to COPII vesicles by binding to a membrane protein receptor.
■ Membrane proteins needed to form COPII vesicles can be retrieved from the cis-Golgi by COPI vesicles. One of the sorting signals that directs membrane proteins into COPI vesicles is a KKXX sequence, which binds to sub-units of the COPI coat.
■ Many soluble ER-resident proteins contain a KDEL sorting signal. Binding of this retrieval sequence to a specific receptor protein in the cis-Golgi membrane recruits missorted ER proteins into retrograde COPI vesicles (see Figure 17-16).
■ COPI vesicles also carry Golgi-resident proteins from later to earlier compartments in the Golgi stack.
■ Soluble and membrane proteins advance through the Golgi complex by cisternal progression, a nonvesicular process of anterograde transport.
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