Evidence for a short form of RGS3 preferentially expressed in the human heart

RGS proteins (regulators of G protein signalling) negatively regulate G pro tein function as GTPase-activating proteins (GAP) for G protein alpha -subu nits. The existence of mRNAs of different size for some of the RGS proteins , e.g. RGS3, suggests that these proteins may exist in isoforms due to alte rnative splicing. We therefore investigated RGS3 mRNA and protein expressio n in different human tissues. Ribonuclease protection assays and Northern b lot analysis showed two specific mRNAs for RGS3 (RGS3L, RGS3S) in human myo cardium, suggesting an additional, N-terminally truncated form of approxima tely 168 aa. When expressed as a recombinant protein RGS3S was recognized a t similar to 23 kDa by an antipeptide antiserum originally raised against a n RGS2 sequence. In membranes of human tissues this antiserum detected spec ific signals for RGS3L (similar to 70 kDa), RGS2 (similar to 30 kDa) and a 25-kDa protein, most likely RGS3S. Both RGS3S mRNA and the 25 kDa protein w ere abundant in human heart, whereas expression in liver, brain and myometr ium was much weaker. To characterize RGS3S functionally, single turnover GT Pase, adenylyl cyclase (AC) and phospholipase C (PLC) activities were deter mined. Both recombinant RGS3S and RGS16 increased P-i release from G(alpha i1) by about 150% and increased GTP- and GTP plus isoprenaline-stimulated A C activity by 20-30% in human left ventricular myocardial membranes. Additi onally, both RGS proteins reduced basal and endothelin-stimulated PLC activ ity in these membranes by about 40%. We conclude that an additional truncated form of RGS3 is expressed in the h uman heart. As described for the full-length protein, RGS3S negatively regu lates the activity of G(i/o)- and G(q)-, but not G(s)-subfamily members.