Photoaffinity labeling of dog pancreas microsomes with 8-azido-ATP inhibits association of nascent preprolactin with the signal sequence receptor complex

The membrane proteins TRAMp and sec61 alpha p may be involved in post-translational transport of presecretory proteins into mammalian microsomes

The presecretory protein ppcecDHFR, a hybrid between preprocecropinA and dihydrofolate reductase, is transported into mammalian microsomes post-translationally, i.e. independent of ribosome and signal recognition particle. Here, the involvement of microsomal proteins in ribonucleoparticle-independent transport of ppcecDHFR was analyzed by transport into trypsin-pretreated microsomes and by transport of a truncated version of ppcecDHFR and subsequent chemical cross-linking. We observed that post-translational transport of ppcecDHFR can occur into microsomes which had been pretreated with trypsin (final concentration, 100 micrograms/ml) and that of the known transport components only TRAMp and sec61 alpha p are still present under these conditions. Furthermore, we found that the truncated ppcecDHFR, ppcecDHFR-98mer', can be cross-linked to 36 kDa microsomal membrane proteins during post-translational transport. Therefore, the two microsomal membrane proteins with molecular masses of about 36 kDa, TRAMp and sec61 alpha p, appear to be involved in the post-translational transport of ppcecDHFR and ppcecDHFR-98mer.

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.

DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibits an early step of protein translocation across the mammalian ER membrane

Protein translocation across the endoplasmic reticulum (ER) membrane of yeast can be inhibited by agents believed to specifically affect the transport of ATP through the membrane (Mayinger, P. and Meyer, D.I. (1993) EMBO J. 12, 659-666), suggesting the involvement of a translocation component in the lumen of the ER that binds ATP. We demonstrate that one of the inhibitors, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), also affects the translocation of proteins into mammalian microsomes. Translocation is blocked at the point of transfer of the nascent chain from the signal recognition particle (SRP) into the ER-membrane. We also confirm that photoaffinity-labelling of microsomes with 8-azido-ATP inhibits the same early step of protein translocation. Since this step is reported to not require ATP, these results raise the possibility that, in both cases, factor(s) other than ATP-binding components of the translocation machinery are perturbed.

Protein targeting to and translocation across the membrane of the endoplasmic reticulum

Several approaches are currently being taken to elucidate the mechanisms and the molecular components responsible for protein targeting to and translocation across the membrane of the endoplasmic reticulum. Two experimental systems dominate the field: a biochemical system derived from mammalian exocrine pancreas, and a combined genetic and biochemical system employing the yeast, Saccharomyces cerevisiae. Results obtained in each of these systems have contributed novel, mostly non-overlapping information. Recently, much effort in the field has been dedicated to identifying membrane proteins that comprise the translocon. Membrane proteins involved in translocation have been identified both in the mammalian system, using a combination of crosslinking and reconstitution approaches, and in S. cerevisiae, by selecting for mutants in the translocation pathway. None of the membrane proteins isolated, however, appears to be homologous between the two experimental systems. In the case of the signal recognition particle, the two systems have converged, which has led to a better understanding of how proteins are targeted to the endoplasmic reticulum membrane.