Vitellin processing and protein synthesis during cricket embryogenesis.

Abstract

At the start of insect embryogenesis most of the protein mass of the egg cytoplasm exists as vitellin (Vt) obtained endocytically during vitellogenesis. Of the new embryo polypeptides (EP) appearing in the egg during embryogenesis, many are synthesized de novo, while, in some species, others derive from developmentally programmed partial proteolysis of Vt. Earlier we showed that by the end of vitellogenesis the two native Vts in Acheta domesticus exist in opposing gradients along the longitudinal axis of the egg. Here we hypothesize that this ooplasmic Vt distribution presents a milieu for Vt processing out of which region-specific regulatory molecules could arise. The metabolic origin and stage-specific patterns of seven predominant EPs (EP 1-7) identified by SDS-PAGE were examined and the results correlated with developmental morphology during the 14 days of embryogenesis. Based on antibody reactivity, peptide mapping and in vitro radiolabeling, we determined that EPs 1-3, 6 and 7 are Vt-derived, while EPs 4 and 5 are produced de novo by the embryo. The five Vt-derived EPs appear during the first 24 h of embryogenesis when migrating cleavage nuclei and associated cytoplasm form the cellular blastoderm, and levels of EPs 4 and 5 increase during days 4-6 of embryogenesis when katatrepsis and yolk mass contraction occur. Positive periodic acid-Schiff staining indicated that EPs 1-3 and their Vt-precursor polypeptides are glycoproteins. This work shows that developmental stage-specific Vt processing occurs during A. domesticus embryogenesis and points next to investigation of the functional significance of Vt cleavage products during development.