The effects of nitric oxide (NO) in diverse cell types in mammals are wide ranging and include control of the immune response, neurotransmission, the cell cycle, and blood vessel relaxation. Thus, many mechanisms are in place to limit the synthesis of NO by nitric oxide synthase (NOS). These include controls at the transcriptional and translational levels, and regulation by covalent modification, protein interactions, and subcellular localization. NO signaling in Drosophila also has diverse regulatory functions, and like mammalian NOS genes, the single Drosophila NOS gene encodes several transcripts that are generated through alternative promoters and splicing. Only a single isoform, dNos1, encodes an active enzyme, whereas the others encode truncated proteins that are catalytically inactive. Stasiv et al. report that the truncated protein dNOS4, which lacks the C-terminal reductase domain, acts as a dominant-negative isoform that suppresses NO production. This dNOS4 isoform is expressed in the developing and adult fly. In the wing imaginal disc of larvae, dNos4 and dNos1 expression overlapped, and both endogenous proteins could be isolated in an immune complex from fly lysate. NOS activity in lysates from transgenic flies ectopically expressing dNOS4 was reduced by about 50% compared with control wild-type flies. Forced expression of dNOS4 also suppressed the antiproliferative effect of NO, resulting in an increased number of cells in the eye. The truncated protein also blocked NOS activity in vitro and when expressed in cultured mammalian cells. The authors propose that truncated NOS isoforms may suppress their active counterparts by forming heterodimers. This additional mechanism reveals the complex nature of NO signal regulation and may likely extend to other organisms.
Y. Stasiv, B. Kuzin, M. Reguski, T. Tully, G. Enikolopov, Regulation of multimers via truncated isoforms: A novel mechanism to control nitric-oxide signaling. Genes Dev. 18, 1812-1823 (2004). [Abstract] [Full Text]