Two overlapping reading frames in a single exon encode interacting proteins - a novel way of gene usage

The >1 kb XL-exon of the rat XL alphas/G alphas gene encodes the 37 kDa XL- domain, the N-terminal half of the 78 kDa neuroendocrine-specific G-protein oc-subunit XL alphas. Here, we describe a novel feature of the XL-exon, th e presence of an alternative >1 kb open reading frame (ORF) that completely overlaps with the ORF encoding the XL-domain. The alternative ORF starts 3 2 nucleotides downstream of the start codon for the XL-domain and is termin ated by a stop codon exactly at the end of the XL-exon. The alternative ORF encodes ALEX, a very basic (pI 11.8), proline-rich protein of 356 amino ac ids. Both XL alphas and ALEX are translated from the same mRNA. Like XLas, ALEX is expressed in neuroendocrine cells and tightly associated with the c ytoplasmic leaflet of the plasma membrane. Remarkably, ALEX binds to the XL -domain of XL alphas. Our results reveal a mechanism of gene usage that is without precedent in mammalian genomes.