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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.

The interaction between the peptide melittin from bee venom with multilamellar liposomes of dipalmitoylphosphatidylcholine or egg yolk phosphatidylcholine has been studied by the method of temperature scanning densitometry, yielding information on the specific volume of the association products and on the changes during the thermotropic transition of the lipids. The effects of the interaction were found to be biphasic with respect to melittin concentration; below 10(-3) mol per mol of phospholipid, an increase in transition temperature, abolition of the pretransition, a reduction in the transition volume of the lipids by about 25%, and a nonadditive increase in apparent specific volume of the complexes were observed. Only minor changes in these parameters could be detected between molar ratios of 10(-3) and 10(-2). Above 1 mol % melittin, the transition temperature decreased and the transition volume approached zero around 10 mol %. Since the effects in the low concentration range cannot be accounted for only by local perturbations around the actual sites of interaction, it is concluded that the interaction involves long range effects of melittin affecting several hundreds of phospholipid molecules. The results are discussed in terms of a phospholipid cluster model, whereby the interaction with melittin leads to a cooperative transition of entire clusters to a state of expanded volume. It is suggested that this transition may be important to the activating effect of melittin on membrane enzymes and to enhanced membrane permeability and lysis.

Honeybee prepromelittin (70 amino acid residues), the precursor of an eukaryotic secretory protein, and a hybrid protein between prepromelittin and mouse dihydrofolate reductase (257 amino acid residues) were expressed in Escherichia coli and characterized with respect to their requirements for transport across the plasma membrane. Both precursor proteins are posttranslationally processed and exported into the periplasm, and they both depend on the membrane potential for this to occur. With respect to dependence on components of the export machinery, however, the two precursor proteins show striking differences: the small precursor protein prepromelittin does not require the function of proteins secA and secY; the large precursor protein prepromelittin-dihydrofolate reductase, on the other hand, depends on both components. The implications of these observations with respect to the mechanisms of protein export in E. coli and of protein import into the endoplasmic reticulum are discussed.