Key Concepts of Section 14.3

• COPII vesicles transport proteins from the rough ER to the cis-Golgi; COPI vesicles transport proteins in the reverse direction (see Figure 14-11).

• 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 14-12). Soluble cargo proteins are probably targeted to COPII vesicles by binding to a membrane protein receptor.

• 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 14-13).

• 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 subunits of the COPI coat. A distinct di-arginine sorting signal operates by a similar mechanism.

• COPI vesicles also carry Golgi-resident proteins from later to earlier compartments in the Golgi complex.

• Soluble and membrane proteins advance through the Golgi complex by cisternal maturation, a process of anterograde transport that depends on resident Golgi enzymes moving by COPI vesicular transport in a retrograde direction.