Organization of translocon complexes in ER membranes

Abstract

Protein translocation in the ER (endoplasmic reticulum) and N-glycosylation are fundamental processes essential for the normal functioning of eukaryotic cells. They are the initial steps in the intracellular pathway that are followed by secretory proteins and membrane proteins of the endomembrane system and the plasma membrane. The translocation and concurrent N-glycosylation of these proteins take place on a large molecular machine, the TC (translocon complex), which is associated with membrane-bound polysomes. Segregation of TCs into a differentiated domain of the ER, the rough ER, may increase the efficiency of protein synthesis on membrane-bound polysomes. Our research is concerned with the assembly, functional organization and dynamics of the TCs in the ER, and their contribution to the functioning and the morphological appearance of this organelle. We hypothesize that the TCs form higher-order structures defining the rough domain of the ER. These structures, which are immobilized or diffuse slowly in the plain of the ER membrane, may be formed and stabilized by mRNAs interconnecting the TCs, by cytoskeletal elements and/or by hypothetical proteins that form links between the TCs. We have established the M3/18 cell line, which expresses the GFP (green fluorescent protein)–Dad1 fusion protein quantitatively and functionally incorporated into the OST (oligosaccharyltransferase). GFP–Dad1 can be used as a reporter molecule for the lateral mobility of the TCs since the OST is tightly associated with the complex. As determined by FRAP (fluorescence recovery after photobleaching), the lateral mobility of GFP–Dad1-tagged TCs was much more restricted than expected from the estimated size of the TC and can be affected by the functional state of the TCs. Currently, we are studying the possible involvement of cytoskeletal elements in the organization of the TCs. Our data suggest that microtubules also play a role in the immobilization of the TCs.