Each organelle of the secretory pathway is required to selectively allow transit of newly
synthesized secretory and plasma membrane proteins and also to maintain a unique set of
resident proteins that define its structural and functional properties. In the case of the
endoplasmic reticulum (ER), residency is achieved in two ways:(a) prevention of residents
from entering newly forming transport vesicles and (b) retrieval of those residents that
escape. The latter mechanism is directed by discrete retrieval motifs: Soluble proteins have …