Sequence and expression of Drosophila Antigen 5-related 2, a new member of the CAP gene family.

A cDNA synthesized and cloned from Drosophila melanogaster mRNA was shown to encode a protein with significant identity to the protein encoded by the Drosophila Antigen 5-related (Agr) gene. Because of its similarity to Agr, this novel cDNA was termed Antigen 5-related 2 (Agr2). The proteins encoded by Agr and Agr2 both belong to the CAP family of proteins, which include the mammalian Cysteine-rich secretory proteins, wasp venom Antigen 5 proteins, and plant group 1 Pathogenesis-related proteins. CAP family proteins share a common core sequence, and most appear to function extracellularly. Agr2 encodes a protein that is 254 amino acids in length. The cDNA contains a short 5' untranslated region (UTR) 36bp in length, and a 3' UTR of 46bp. The protein encoded by Agr2 has 48% identity and 61% similarity to that encoded by Agr. The Agr2 gene was localized to region 12F of the X chromosome and probably lies near the Agr gene. Agr2 RNA was approx. 950nt in length and was most abundant during the larval period, although transcripts could also be detected at other developmental stages. Transcription of Agr2 is initiated in the embryo within the midgut epithelium near the site where the third midgut constriction will form. As embryogenesis proceeds, Agr2 expression expands to form a domain located just posterior to the third midgut constriction. This domain corresponds to a single loop of the midgut in late-stage embryos. Based on its expression pattern and its similarity to other CAP family proteins, the Agr2 protein is hypothesized to function either as a novel type of protease inhibitor or as an antimicrobial protein.