THE DROSOPHILA LSP-1-BETA GENE A STRUCTURAL AND PHYLOGENETIC ANALYSIS

In Drosophila melanogaster, metamorphosis and reproduction are thought to be supported in large by two immunologically distinct hexameric st orage proteins (hexamerins), larval serum protein 1 (LSP-1), a mixed h examer of three closely related subunits, Lsp-1(alpha, beta and gamma) and larval serum protein 2 (LSP-2), a homohexamer of Lsp-2 subunits. To understand the structural and functional differences between these two storage hexamers, the nucleotide sequence of the coding region of the Lsp-1 beta gene was determined for comparison with LSP-2 and a num ber of other arthropod hexamerins. The G+C content of the coding seque nce is 55%, with 92.8% of the codons containing G or C in the third po sition. Conceptual translation of the Lsp-1 beta open reading frame re vealed a 789-amino-acid polypeptide of 94465 Da. The amino acid sequen ce of Lsp-1 beta is 65.8% identical to that of calliphorin, the major hexamerin of the blowfly, Calliphora vicina, and only 35.2% identical to Drosophila Lsp-2. This greater similarity to calliphorin is also re flected in high aromatic amino acid and methionine contents, in contra st to LSP-2 which is enriched to a lesser extent only in aromatic amin o acids. Lsp-1 beta is also more closely related to calliphorin with r espect to the protein domain structure, the presence of a single intro n in its gene, and the absence of glycosylation sites. However, phylog enetic analysis based on multiple alignments revealed that LSP-1/calli phorin and LSP-2 form a distinct dipteran clade whose members are more similar to each other than to any previously sequenced lepidopteran h examerin or arthropod hemocyanin.