An unusual pattern in a nucleic acid or protein sequence or a region of strong similarity shared by two or more sequences may have biological significance. It is therefore desirable to know whether such a pattern can have arisen simply by chance. To identify interesting sequence patterns, appropriate scoring values can be assigned to the individual residues of a single sequence or to sets of residues when several sequences are compared. For single sequences, such scores can reflect biophysical properties such as charge, volume, hydrophobicity, or secondary structure potential; for multiple sequences, they can reflect nucleotide or amino acid similarity measured in a wide variety of ways. Using an appropriate random model, we present a theory that provides precise numerical formulas for assessing the statistical significance of any region with high aggregate score. A second class of results describes the composition of high-scoring segments. In certain contexts, these permit the choice of scoring systems which are "optimal" for distinguishing biologically relevant patterns. Examples are given of applications of the theory to a variety of protein sequences, highlighting segments with unusual biological features. These include distinctive charge regions in transcription factors and protooncogene products, pronounced hydrophobic segments in various receptor and transport proteins, and statistically significant subalignments involving the recently characterized cystic fibrosis gene.

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Methods for assessing the statistical significance of molecular sequence features by using general scoring schemes

The renin-angiotensin system plays a critical role in blood pressure control and body fluid and electrolyte homeostasis. Besides angiotensin (Ang) II, other Ang peptides, such as Ang III [Ang-(2-8)], Ang IV [Ang-(3-8)], and Ang-(1-7) may also have important biological activities. Ang-(1-7) has become an angiotensin of interest in the past few years, because its cardiovascular and baroreflex actions counteract those of Ang II. Unique angiotensin-binding sites specific for this heptapeptide and studies with a selective Ang-(1-7) antagonist indicated the existence of a distinct Ang-(1-7) receptor. We demonstrate that genetic deletion of the G protein-coupled receptor encoded by the Mas protooncogene abolishes the binding of Ang-(1-7) to mouse kidneys. Accordingly, Mas-deficient mice completely lack the antidiuretic action of Ang-(1-7) after an acute water load. Ang-(1-7) binds to Mas-transfected cells and elicits arachidonic acid release. Furthermore, Mas-deficient aortas lose their Ang-(1-7)-induced relaxation response. Collectively, these findings identify Mas as a functional receptor for Ang-(1-7) and provide a clear molecular basis for the physiological actions of this biologically active peptide.

https://www.pnas.org/content/pnas/100/14/8258.full.pdf

Angiotensin-(1-7) is an endogenous ligand for the G protein-coupled receptor Mas.

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Physiology of local renin-angiotensin systems.

Angiotensin II is an important effector molecule controlling blood pressure and volume in the cardiovascular system. Its importance is manifested by the efficacy of angiotensin-converting enzyme inhibitors in the treatment of hypertension and congestive heart failure. Angiotensin II interacts with two pharmacologically distinct subtypes of cell-surface receptors, AT1 and AT2. AT1 receptors seem to mediate the major cardiovascular effects of angiotensin II. Here we report the isolation by expression cloning of a complementary DNA encoding a unique protein with the pharmacological specificity of a vascular AT1 receptor. Hydropathic modelling of the deduced protein suggests that it shares the seven-transmembrane-region motif with the G protein-coupled receptor superfamily. Knowledge of the AT1 receptor primary sequence should now permit structural analysis, definition of the angiotensin II receptor gene family and delineation of the contribution of AT receptors to the genetic component of hypertension.

Isolation of a cDNA encoding the vascular type-1 angiotensin II receptor

The role of a local angiotensin system in the vascular response to arterial injury was investigated by administering the angiotensin-converting enzyme (CE) inhibitor cilazapril to normotensive rats in which the left carotid artery was subjected to endothelial denudation and injury by balloon catheterization. In control animals, by 14 days after balloon injury, the processes of smooth muscle cell (SMC) proliferation, migration of SMCs from the media to the intima, and synthesis of extracellular matrix produced marked thickening of the intima, with reduction of the cross-sectional area of the lumen. However, in animals that received continuous treatment with the CE inhibitor, neointima formation was decreased (by about 80 percent), and lumen integrity was preserved. Thus, the angiotensin-converting enzyme may participate in modulating the proliferative response of the vascular wall after arterial injury, and inhibition of this enzyme may have therapeutic applications to prevent the proliferative lesions that occur after coronary angioplasty and vascular surgery.

https://science.sciencemag.org/content/sci/245/4914/186.full.pdf

Inhibitors of angiotensin-converting enzyme prevent myointimal proliferation after vascular injury.

The class of receptors coupled to GTP-binding proteins share a conserved structural motif which is described as a 'seven-transmembrane segment'1 following the prediction that these hydrophobic segments form membrane-spanning α-helices. Identified examples include the mammalian opsins2, α1-, α2-, β1- and β2-adrenergic receptors3, the muscarinic receptor family4,5, the 5-HT1C-receptor6, and the substance-K receptor7. In addition, two mammalian genes have been identified that code for predicted gene products with sequence similarity to these receptors, but whose ligand specificity is unknown namely, G21 (ref. 8) and the mas oncogene9. The mas oncogene shows the greatest sequence similarity to the substance-K receptor, and on this basis it was predicted that it would encode a peptide receptor with mitogenic activity which would act through the inositol lipid signalling pathways10,11. The mas oncogene product was transiently expressed in Xenopus oocytes, and stably expressed in a transfected mammalian cell line. The results demonstrate that the mas gene product is a functional angiotensin receptor.

T. R. Jackson

Papers

The mas oncogene encodes an angiotensin receptor