Differential vitellin polypeptide processing in insect embryos

This review focuses on the current status of knowledge regarding the process of vitellogenin (Vg) uptake and vitellin (Vt) storage in insect oocytes. We also consider the overall morphology of the yolk sac as an embryonic organ allowing for both temporal and spatial differentiation of yolk degradation to provide the primary food supply for the embryo. In this context we describe the evidence that demonstrates the occurrence of Vt polypeptide processing and how it may be affected by maternally derived proteases stored in the yolk granules and by the ubiquitin–proteasome system in the cytosol. The extent of Vt polypeptide processing induced by these proteases will be correlated with the structural modifications affecting yolk granules and vitellophages in developing insect embryos. To accomplish these goals the ultrastructural and cytochemical composition of yolk granules during vitellogenesis and embryogenesis will be reviewed. Last but not the least, our current understanding of the role played by acidification of yolk granules in the activation of maternally derived proteases will also be examined.