The fate of Xenopus and locust vitellogenins made in Xenopus oocytes. An export-import processing model.

Abstract

Xenopus liver or locust fat body RNA injected into Xenopus oocytes directs the formation of a high‐molecular‐weight species which is found transiently within the oocyte in a membranous vesicle fraction. The conversion of such internally generated Xenopus vitellogenin to yolk platelet proteins is reduced by the presence in the incubation medium of anti‐vitellogenin antibodies, or of tunicamycin, or of a mixture of colchicine and cytocholasin. but neither export nor conversion is blocked by removal of the follicular layers which surround the oocyte. Moreover, surrounding mRNA injected defolliculated oocytes with an excess of small defolliculated oocytes leads to a reduction in lipovitellin accumulation in the cells making vitellogenin and the appearance of lipovitellin in the platelets of the uninjected feeder cells.We propose that newly‐made locust and Xenopus vitellogenins are sequestered in vesicles and are then secreted by the oocyte, the locust species being further modified by cleavage just before export. Thus processing of heterologous yolk precursors follows the pathway characteristic of the cell type used to prepare the donor RNA. We suggest that the import mechanism discriminates between the exported locust and frog proteins, and that only the Xenopus vitellogenin subsequently enters the oocyte, where it is converted to lipovitellin and phosvitin by an enzyme present in the yolk platelets. Thus we explain the apparently paradoxical observations that internally generated and externally supplied frog vitellogenin are both converted to yolk platelet proteins, whilst injected vitellogenin is not.