ALIGNMENT OF CONDUITS FOR THE NASCENT POLYPEPTIDE-CHAIN IN THE RIBOSOME-SEC61 COMPLEX

An oligomer of the Sec61 trimeric complex is thought to form the prote in-conducting channel for protein transport across the endoplasmic ret iculum. A purified yeast Sec61 complex bound to monomeric yeast riboso mes as an oligomer in a saturable fashion. Cryo-electron microscopy of the ribosome-Sec61 complex and a three-dimensional reconstruction sho wed that the Sec61 oligomer is attached to the large ribosomal subunit by a single connection. Moreover, a funnel-shaped pore in the Sec61 o ligomer aligned with the exit of a tunnel traversing the large ribosom al subunit, strongly suggesting that both structures function together in the translocation of proteins across the endoplasmic reticulum mem brane.