The sequestration, processing and retention of honey-bee promelittin made in amphibian oocytes

Messenger RNAs from one kind of secretory cell can be introduced into the cytoplasm of another: the heterologous proteins formed by the recipient cell are usually processed and topologically segregated in the manner characteristic of the donor cell. Xenopus oocytes injected with honey-bee venom gland RNA provide some support for this generalization, but also reveal important exceptions to it. Thus, the frog cell makes a small polypeptide whose partial sequence matches perfectly that of insect promelittin, except that the product formed in oocytes ends at the C terminus with a glycine as opposed to a glutamine amide residue. N-terminal heterogeneity is seen in protoxin made in oocytes and venom gland cells, and species shorter by two residues are seen in both tissues. We suggest that the oocyte contains a dipeptidylpeptidase. Promelittin made in oocytes is barely detectable in the cytosol but is found associated with a vesicle fraction which also contains some newly synthesized endogenous oocyte proteins. The association with vesicles is long-lasting; thus promelittin is retained slightly more efficiently than sequestered oocyte proteins, and an incubation period of about two weeks is required to reduce by half the amount of these endogenous vesicle proteins. Thus neither promelittin nor any products derived from it are secreted rapidly. Gel analysis fails to reveal promelittin in the medium surrounding the oocyte, although traces can be detected by assaying for a characteristic heptapeptide. Such small amounts could result from slow secretion or leakage. Melittin could not be detected by gel analysis or peptide assay. The retention of the honey-bee protein within the frog cell is discussed in terms of the specificity of the processing systems and secretory pathways of venom gland cells and oocytes. We suggest that whilst some export mechanisms function efficiently in a wide variety of cells, others do not, and may even be restricted to specific cell types.