Abstract

In eukaryotic cells the secretory pathway consists of series of compartments that modify, transport and sort secretory cargo. Coat protein complexes are responsible for vesicle formation during the transport events and recognition of signals exposed to those complexes insure sorting and targeting of proteins between the various compartments. Cargo transport molecules bear at their C-terminus (cytoplasmic) short motifs used to direct them to and or retain them in specific compartments. For example DXE motif facilitates ER export of post ER resident proteins and di Phenylalanine (diPhe) motif drives anterograde ER to Golgi transport of cargo receptors.
In order to recycle some organellar components and cargo selecting molecules while the secretory cargo advances the transport within the secretory pathway has to be bidirectional . Anterograde transport is catalysed by COPII coat protein complex and retrograde transport involves COPI complex. Some proteins bear determinants allowing them to undergo transport in both directions. In many instances the diPhe motif of some cargo receptors cycling between the ER and the Golgi apparatus coexists with a diLys motif : it has been proposed that diPhe triggers anterograde transport between ER and Golgi (binding to COPII) while diLysine (see Functional site di Lysine ER retrieving signal and ELM TRG_ER_diLysine_1) triggers return to the ER. The members of the p24 family are examples of diPhe bearing type I integral membrane proteins : they are involved in vesicle trafficking and reside at the cis Golgi network under steady state conditions. Their cytoplasmic tail mediates the recruitment of ARF1-GDP (ADP-ribosylation factor 1) to Golgi membranes (PMIDPMID:11726511), subsequently membrane bound ARF1-GTP triggers the recruitment of coatomer.