ALTERNATIVELY SPLICED HUMAN TYPE-1 ANGIOTENSIN-II RECEPTOR MESSENGER-RNAS ARE TRANSLATED AT DIFFERENT EFFICIENCIES AND ENCODE 2 RECEPTOR ISOFORMS

The peptide hormone angiotensin II (AngII) plays a principal role in r egulating blood pressure and fluid homeostasis. Most of its known effe cts are mediated by a guanine nucleotide-regulatory protein (G protein )-coupled receptor pharmacologically defined as the type-1 AngII recep tor or AT1. Characterization of cDNA and genomic clones shows that the human AT1 gene contains five exons and encodes two receptor isoforms as a result of alternative splicing. Exon 5 contains the previously ch aracterized open reading frame for AT1, and exons 1 to 3 are alternati vely spliced upstream of it to generate several mRNA species, while tr anscripts containing exon 4 are of minor abundance. In an in vitro tra nslation system, the presence of exon 1 was found to be extremely inhi bitory to translation, probably because it can form a stable secondary structure at the RNA level. The alternatively spliced second exon als o had a strong inhibitory effect on translation, presumably because it contains a minicistron commencing with an ATG in an optimal context f or translation initiation. Exon 2 was similarly inhibitory to protein production in transfected cells, but exon 1 was found to enhance prote in synthesis in this system. Transcripts containing exon 3 and 5, whic h comprise up to one-third of AT1 mRNAs in all tissues examined, encod e a receptor with an amino-terminal extension of 32-35 amino acids. Th ese transcripts were translated into a larger receptor isoform in vitr o and produced a functional receptor with normal ligand binding and si gnaling properties in transfected cells.