Co-translational translocation at the endoplasmic reticulum (ER) plays a critical role in the targeting of both soluble and membrane proteins to their correct intra- and intercellular compartments. We are studying the 3D architecture of the ribosome-Sec61p complex (translocon), with the aim of understanding the physical mechanisms of gating and transport. To this end, we are using single particle electron cryo-microscopy and 3D reconstruction of frozen hydrated channel complexes, to obtain interpretable and biologically relevant maps.

Previously, we have shown that both co- and post-translational translocation utilize a common central channel comprised of a ring-like Sec61p oligomer. Moreover, this channel morphology is shared with the related Sec YE complex from B. subtilus. Mass analysis, volume calculations and ribosome binding experiments suggest a stoichiometry of 3-4 Sec61p heterotrimers per ring. We currently favor 4 copies of the Sec61p complex per channel, as projection maps demonstrate 4 nearly equi-spaced peaks around the central pore.