Lipophorin receptor from the silkworm, Bombyx mori: cDNA cloning, genomic structure, alternative splicing and isolation of a new isoform

Ravikumar Gopalapillai, Keiko Kadono-Okuda, Kozo Tsuchida, Kimiko Yamamoto, Junko Nohata, Masahiro Ajimura, and Kazuei Mita

Seri-biotech Research Lab, Central Silk Board, Bangalore, Karnataka 560 035

Corresponding Author: ravi_1_98{at}yahoo.com

The cDNA and genomic structure of a lipophorin receptor (BmLpR) from the silkworm, Bombyx mori, indicated the presence of four isoforms designated as LpR1, LpR2, LpR3 and LpR4. The deduced amino acid sequence of each isoform showed five functional domains that are homologous to vertebrate very low-density lipoprotein receptor (VLDLR). All four isoforms seem to have originated from a single gene by alternative splicing, and were differentially expressed in a tissue and stage-specific manner. BmLpR1 harbored an additional twenty-seven amino acids in the O-linked sugar domain resulting in an extra exon. The silkworm BmLpR gene consisted of 16 exons separated by 15 introns spanning over 122 kb and was at least three times bigger than the human VLDLR gene. Surprisingly, one of the isoforms, LpR4, was specifically expressed in the brain and ventral nervous system. Additionally, it had a unique cytoplasmic tail, leading to the proposition that it represents a new candidate LpR for possible brain related function(s). This is the first report on the genomic characterization of an arthropod lipoprotein receptor gene and the identification of a brain-specific receptor variant from a core member of the LDLR family in invertebrates.

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