The membrane proteins TRAMp and sec61αp 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.