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 storage proteins (hexamerins), larval serum protein 1 (LSP-1), a mixed hexamer 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 number of other arthropod hexamerins. The G + C content of the coding sequence is 55%, with 92.8% of the codons containing G or C in the third position. Conceptual translation of the Lsp-1 beta open reading frame revealed a 789-amino-acid polypeptide of 94465 Da. The amino acid sequence 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 reflected in high aromatic amino acid and methionine contents, in contrast to LSP-2 which is enriched to a lesser extent only in aromatic amino acids. Lsp-1 beta is also more closely related to calliphorin with respect to the protein domain structure, the presence of a single intron in its gene, and the absence of glycosylation sites. However, phylogenetic analysis based on multiple alignments revealed that LSP-1 calliphorin and LSP-2 form a distinct dipteran clade whose members are more similar to each other than to any previously sequenced lepidopteran hexamerin or arthropod hemocyanin.