Cloning and functional characterization of a putative sodium channel auxiliary subunit gene from the house fly (Musca...

The functional expression of cloned Drosophila melanogaster and house fly (Musca domestica) voltage-sensitive sodium channels in Xenopus oocytes is enhanced, and the inactivation kinetics of the expressed channels are accelerated, by coexpression with the tipE protein, a putative sodium channel auxiliary subunit encoded by the tipE gene of D. melanogaster. These results predict the existence of a tipE ortholog in the house fly. Using a PCR-based homology probing approach, we isolated cDNA clones encoding an ortholog of tipE (designated Vssc beta) from adult house fly heads. Clones comprising 3444 bp of cDNA sequence contained a 1317 bp open-reading frame encoding a 438 amino acid protein. The predicted Vssc beta protein exhibited 72% amino acid sequence identity to the entire D. melanogaster tipE protein sequence and 97% identity within the two hydrophobic segments identified as probable transmembrane domains. Coexpression of Vssc beta with the house fly sodium channel alpha subunit (Vssc1) in oocytes enhanced the level of sodium current expression five-fold and accelerated the rate of sodium current inactivation 2.2-fold. Both of these effects were significantly larger in magnitude than the corresponding effects of the D. melanogaster tipE protein on the expression and kinetics of Vssc1 sodium channels. These results identify a second example of a putative sodium channel auxiliary subunit from an insect having functional but not structural homology to vertebrate sodium channel beta subunits.