XLαs, the extra-long form of the α-subunit of the Gs G protein, is significantly longer than suspected, and so is its companion Alex

Abstract

Because of the use of alternate exons 1, mammals express two distinct forms of Gsα-subunits: the canonical 394-aa Gsα present in all tissues and a 700+-aa extra-long αs (XLαs) expressed in a more restricted manner. Both subunits transduce receptor signals into stimulation of adenylyl cyclase. The XL exon encodes the XL domain of XLαs and, in a parallel ORF, a protein called Alex. Alex interacts with the XL domain of XLαs and inhibits its adenylyl cyclase-stimulating function. In mice, rats, and humans, the XL exon is thought to contribute 422.3, 367.3, and 551.3 codons and to encode Alex proteins of 390, 357, and 561 aa, respectively. We report here that the XL exon is longer than presumed and contributes in mice, rats, and humans, respectively, an additional 364, 430, and 139 codons to XLαs. We called the N-terminally extended XLαs extra-extra-long Gsα, or XXLαs. Alex is likewise longer. Its ORF also remains open in the 5′ direction for ≈2,000 nt, giving rise to Alex-extended, or AlexX. RT-PCR of murine total brain RNA shows that the entire XXL domain is encoded in a single exon. Furthermore, we discovered two truncated forms of XXLαs, XXLb1 and XXLb2, in which, because of alternative splicing, the Gsα domain is replaced by different sequences. XXLb proteins are likely to be found as stable dimers with AlexX. The N-terminally longer proteins may play regulatory roles.