alpha-Crystallin protein cognates in eggs of the moth, Plodia interpunctella: possible chaperones for the follicular epithelium yolk protein

alpha-Crystallin protein cognates were found in germ cells of the Indianmeal moth, Plodia interpunctella (Shirk and Zimowska, 1997). A cDNA clone of 674 bp with a single open reading frame was isolated for a 25 000 molecular weight polypeptide member of this family, alpha CP25, and a single transcript of approximately 700 bp was found in the ovary of vitellogenic females. Both the DNA sequence and predicted amino acid sequence showed considerable homology with the embryonic lethal gene, l(2)efl, in Drosophila melanogaster. In addition to the sequence for l(2)efl, the predicted amino acid sequence for alpha cp25 also showed significant sequence similarity with the alpha-crystallin A chain polypeptides from the lenses of vertebrate eyes. An N-terminal hydrophobic aggregation site and a C-terminal protective binding site common to alpha-crystallin proteins were present in the predicted alpha cp25 and l(2)efl amino acid sequences, while only the C-terminal protective binding site was present in the small heat shock protein sequences from D. melanogaster. On the other hand, the cDNA sequence for alpha cp25 showed more similarity to small heat shock proteins in D. melanogaster. This evidence suggests that although the alpha-crystallin protein cognates in P. interpunctella evolved from a gene common with small heat shock protein genes, the amino acid sequence has converged on a structure similar to that of alpha-crystallin proteins. Native immunoblot analysis showed that the alpha-crystallin proteins formed high molecular weight complexes with the follicular epithelium yolk protein (FEYP) but not vitellin in yolk. An electroblot binding assay was used to show that the germ-cell alpha-crystallins of P. interpunctella bind specifically with the FEYP and that the binding was reversible in the presence of ATP or low pH. This evidence in conjunction with the evidence that the alpha-crystallins and FEYP form a stable complex that co-purifies from native egg proteins suggests that the alpha-crystallin cognates function as chaperones for the follicular epithelium yolk proteins in the embryos of P. interpunctella.