Strukturelle Dynamik und Regulation des Sec61p-Komplexes
Zusammenfassung der Projektergebnisse
The electrophysiological properties of active Sec61 complexes in the endoplasmic reticulum (ER) membrane of canine pancreas- and yeast-cells were investigated by the planar bilayer technique. After activation of the Sec61 complexes by various substrate polypeptides, two regimes of transient channel openings were observed. These corresponded to internal mean pore diameters of 1.2 and 2.2 nm. The various substrate polypeptides determined the respective pore size distribution. Thus the Sec61 complex contains a substrate-activated channel that, once active, fluctuates between two distinct conformations. In the one state small pores exist, in the other state, a single channel pore with roughly doubled pore size is formed. The size of these channel pores is only compatible with an oligomeric structure of the Sec61 complex. In addition, the channel is regulated by ribosomes, calcium-calmodulin, and by the Hsp70 chaperone BiP on the lumenal face of the membrane. We propose that these interactions are important factors in preventing the uncontrolled efflux of calcium and other small ions from this calcium storage compartment. This is particularly important in the light of the structural data that argue against a tight seal between the ribosomal tunnel exit and the Sec61 complex.
