Sequestered and Injected Vitellogenin

Vitellogenin (1, 2) is a sex-limited phosphoprotein secreted by the liver in Xenopus females (3, 4) and selectively transferred via the circulatory system to growing oocytes, within which it is converted into the yolk proteins lipovitellin and phosvitin (3, 5, 6) . Selective uptake (7) and conversion (8) of vitellogenin can also take place in isolated oocytes . The available evidence indicates that vitellogenin is incorporated by a micropinocytotic mechanism and that the derived pinosomes subsequently fuse within the cortex of the oocyte and give rise to yolk platelet primordia (5, 9, 10) . The conversion to lipovitellin and phosvitin involves a macromolecular restructuring within yolk platelet primordia rather than breakdown and resynthesis (8, 11), but the conversion mechanism remains unknown . Protein synthesized by the developing oocyte appears to undergo some degree of turnover whereas sequestered vitellogenin does not (8) . This would imply that vitellogenin injected into the cytoplasm of the oocyte might eventually undergo breakdown rather than conversion unless it could traverse the membranes of the pinosomes and primordial yolk platelets . The present report documents a test of this assumption and indicates that indeed vitellogenin injected into, rather than sequestered by, oocytes is not converted to lipovitellin and phosvitin, but instead appears to be simply catabolized .