Acidification of yolk granules in Blattella germanica eggs coincident with proteolytic processing of vitellin

In eggs of the cockroach Blattella germanica, vitellin (Vt) utilization by the embryo is initiated at day 4 postovulation by the proteolytic processing of its three subunits to a specific set of peptides. A report from our laboratory (Nordin et al.: Archives of Insect Biochemistry and Physiology 15:119, 1990) described a yolk proteinase, activated at days 3-4, which processes the Vt. Further investigation of this event has focused on the yolk granules. Granules from eggs 4-6 days postovulation contained a significant subpopulation which accumulated high concentrations of the dye acridine orange (AO), a fluorescent probe of vesicle acidification, while those from eggs 0-3 days postovulation did not. AO accumulation was caused by proton translocation and was not due to dye binding or a Donnan equilibrium. The temporal correlation of granule acidification with Vt processing suggests a role for this event in yolk proteinase activation in B. germanica. This hypothesis was supported by the finding that incubation of yolk from freshly ovulated eggs in vitro at pH of 5 and below resulted in Vt processing. Yolk granules of the blowfly Phormia regina also became acidified but this occurred in the oocyte prior to egg deposition