Ostrich (Struthio camelus) eggshell matrix contains two different C-type lectin-like proteins. Isolation, amino acid sequence, and posttranslational modifications.

In contrast to chicken and goose, the ostrich calcified eggshell layer matrix contained two different C-type lectin-like proteins as major components. These proteins, named struthiocalcin-1 (SCA-1) and struthiocalcin-2 (SCA-2), were isolated and their amino acid sequence was determined. SCA-1 clearly was the ortholog of goose eggshell ansocalcin. Its amino acid sequence had the same length as that of ansocalcin (132 aa) and showed 65% sequence identity with the goose eggshell protein compared to 41% with chicken eggshell ovocleidin-17. Furthermore, as ansocalcin and unlike ovocleidin-17, it contained an additional, seventh, cysteine that was, however, located close to the C-terminus of SCA-1 and not in the N-terminal third of the sequence as in ansocalcin. SCA-2 showed features of both ansocalcin and chicken eggshell ovocleidin-17 (OC-17). Its sequence was 46% identical to that of ansocalcin and 36% identical to OC-17. It contained the same stretches of negatively charged amino acids conserved in ansocalcin and SCA-1, which are absent in OC-17. On the other hand, its length of 142 amino acids was identical to that of OC-17 and it contained only the usual set of six cysteines conserved in most C-type lectin-like proteins. The presence of three phosphorylated serines located at exactly the same region of the sequence as the two phosphoserines of OC-17 further stressed the similarity between SCA-2 and OC-17.