Signals and mechanisms for protein retention in the endoplasmic reticulum

After their co-translational insertion into the ER lumen or the ER membrane , most proteins are transported via the Golgi apparatus downstream on the s ecretory pathway while a few protein species are retained in the ER. Polype ptide retention in the ER is either signal-independent or depends on specif ic retention signals encoded by the primary sequence of the polypeptide, A first category, i.e. the newly synthesized polypeptides that are unable to reach their final conformation, are retained in the ER where this quality c ontrol generally results in their degradation. A second category, namely th e ER-resident proteins escape the bulk flow of secretion due to the presenc e of a specific N- or C-terminal signal that interacts with integral membra ne or soluble receptors, ER retention of soluble proteins mediated by eithe r KDEL, HDEL or related sequences and membrane receptors has been relativel y well characterized in plants. Recent efforts have aimed at a characteriza tion of the retention signal(s) of type I membrane proteins in the plant ER .