Import of honeybee prepromelittin into the endoplasmic reticulum. Requirements for membrane insertion, processing, and sequestration.

Honeybee prepromelittin is correctly processed and imported by dog pancreas microsomes. Membrane insertion of prepromelittin, assayed as signal sequence removal by signal peptidase, is not dependent on signal recognition particle and docking protein. However, a previously uncharacterized proteinaceous component of the microsomal membrane is required for completion of membrane transfer of promelittin. Furthermore, membrane insertion of prepromelittin is not coupled to translation. These data suggest the signal sequence, in addition to its role in membrane recognition, has a more general function for membrane insertion, cotranslational import of proteins is not an intrinsic feature of microsomes, and at least in certain cases, proteinaceous membrane components are involved in membrane transfer.