The membrane proteins TRAMp and sec61αp may be involved in post‐translational transport of presecretory proteins into mammalian microsomes

Abstract

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, 100μg/ml) and that of the known transport components only TRAMp and sec61αp 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 secolαp, appear to be involved in the post‐translational transport of ppcecDHFR and ppcecDHFR‐98mer.