The mechanism by which proteins are exported from the ER is controversial. Supporters of the `bulk flow' model contend that ER export is a default mechanism that does not require sorting signals in the cargo. Increasing evidence, however, indicates that at least some membrane proteins contain specific ER-export motifs. ERGIC-53 and p24, for example, contain a C-terminal FF motif necessary for efficient ER export. Hans-Peter Hauri and co-workers have used ERGIC-53 mutants to define the signals required for efficient ER export: a single F/Y residue at position -2; an II or LL sequence; or a single V residue at position -1. In fact, a -1V can even accelerate ER export of an otherwise slowly exported CD4 construct. These motifs are common in type I membrane proteins. Moreover, the authors demonstrate that they can bind to COPII components (the coat proteins involved in budding of vesicles from the ER) and...

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