A STABLY FOLDED PRESECRETORY PROTEIN ASSOCIATES WITH AND UPON UNFOLDING TRANSLOCATES ACROSS THE MEMBRANE OF MAMMALIAN MICROSOMES

The presecretory protein ppcecDHFR, a hybrid between preprocecropin A and dihydrofolate reductase, is transported into mammalian microsomes post-translationally, i.e. independently of ribosome and signal recognition particle. Upon staging the transport process, stably folded ppcecDHFR bound to mammalian microsomes and subsequently translocated across the membrane. Membrane association depended on the signal peptide but involved neither ATP nor an N-ethylmaleimide-sensitive microsomal protein. Membrane insertion of bound ppcecDHFR did not necessitate unfolding of the DHFR domain but depended on ATP and an N-ethylmaleimide-sensitive microsomal protein. Completion of translocation relied on unfolding of the DHFR domain. Thus mammalian microsomes have the capability of transporting a bound and folded precursor protein, i.e. to trigger unfolding of a precursor protein on the membrane surface.