BIP MAINTAINS THE PERMEABILITY BARRIER OF THE ER MEMBRANE BY SEALING THE LUMENAL END OF THE TRANSLOCON PORE BEFORE AND EARLY IN TRANSLOCATION

Secretory proteins are cotranslationally translocated across the mamma lian ER membrane through an aqueous pore in the translocon while the p ermeability barrier is maintained by a tight ribosome-membrane junctio n. The lumenal end of the pore is also blocked early in translocation. Extraction of soluble lumenal proteins from microsomes and reconstitu tion with purified proteins demonstrate, by fluorescence collisional q uenching, that BiP seals the lumenal end of this pore. BiP also seals translocons that are assembled but are not engaged in translocation. T hese ribosome-free translocons have smaller pores (9-15 Angstrom diame ter versus 40-60 Angstrom in functioning translocons) and are generate d when ribosomes dissociate from functioning translocons with large po res. BiP therefore maintains the permeability barrier by sealing both nontranslocating and newly targeted translocons.