The Xenopus Oocyte as a Surrogate Secretory System The Specificity of Protein Export

Abstract

Combining mRNA from 1 kind of secretory cell with the cytoplasm of another such cell can reveal the nature and specificty of protein export mechanisms. mRNA from secretory cells of chickens, rats, mice, frogs, guinea pigs, locusts and barley plants, when injected into Xenopus oocytes, direct the synthesis and export of proteins. Chicken ovalbumin, Xenopus albumin, mouse thyroid-stimulating hormone [thyrotropin], locust vitellin and guinea pig milk proteins were identified using specific antibodies; chicken lysozyme and ovomucoid, rat albumin, Xenopus vitellogenin and rat seminal vesicle basic proteins were identified provisionally from the MW. Certain endogenous proteins are sequestered and secreted, although most oocyte proteins are not exported. Similarly the major polyoma viral protein and the SV-40 and polyoma tumor antigens are retained within the oocyte. Radioactive protein exported by oocytes that are programmed with chicken oviduct or Xenopus liver RNA are not reexported in detectable amounts when injected into fresh oocytes; there is also no secretion of chicken oviduct or guinea pig mammary gland primary translation products prepared using wheat germ extracts. The export of secretory proteins from oocytes cannot be explained by leakage and may require a cotranslational event. The secretory system of the oocyte is neither cell-type nor species-specific yet is highly selective. Apparently the oocyte can be used as a general surrogate system for the study of gene expression, from transcription through translation to the final subcellular or extracellular destination of the processed protein.