We have identified a 52 kDa protein, which is a potent substrate for cholera toxin-dependent ADP-ribosylation, in the compound eye preparation of the fruit fly, Drosophila melanogaster. We find that the 52 kDa protein is a glycoprotein and a Ca2+ binder bearing a high content of leucine, serine and glycine. By microsequencing we determined its 13 N-terminal sequence, AYL*PIDLNQLAK, with the asterisk representing an ambiguous signal. In order to study further the 52 kDa protein we have raised a polyclonal antibody against a synthetic oligopeptide representing the N-terminal 13 residues of the 52 kDa protein. By immunogold labelling with the antibody, the epitopes were localized at the EM level to the laminated corneal lens. The number of the gold particles per microns2 in the electron-dense layer of the corneal lens was 2.5 times higher than that of the electron-lucent layer. The pattern of the 52 kDa protein distribution in the corneal lens suggests that the 52 kDa protein is the major protein component that participates in the pattern formation of the alternate refractive indices of the D. melanogaster corneal lens. An X-ray dispersion analysis in situ revealed that the laminated corneal lens contained a higher concentration of Ca2+, supporting the hypothesis that the 52 kDa protein binds Ca2+ in vivo. To the best of our knowledge, this is the first report that identifies the protein entity of an arthropod corneal lens. We propose to designate this 52 kDa protein drosocrystallin.

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