Angiogenin

Angiogenin (Ang) is a small polypeptide that is implicated in angiogenesis (formation of new blood vessels) in tumor growth. However, angiogenin is unique among the many proteins that are involved in angiogenesis in that it is also an enzyme with an amino acid sequence 33% identical to that of bovine pancreatic ribonuclease (RNase) A). Moreover, although Ang has the same general catalytic properties as RNase A - it cleaves preferentially on the 3' side of pyrimidines and follows a transphosphorylation/hydrolysis mechanism - its activity differs markedly both in magnitude and in specificity.

Although angiogenin contains counterparts for the key catalytic residues of bovine pancreatic RNase A, it cleaves standard RNase substrates 105 - 106 times less efficiently than does RNase A. Despite this apparent weakness, the enzymatic activity of Ang appears to be essential for biological activity: replacements of important active site residues invariably diminish ribonuclease and angiogenesis activities in parallel, and a substitution that increases enzymatic activity also enhances angiogenic potency.

Angiogenin may function as a tRNA-specific ribonuclease that binds to actin on the surface of endothelial cells; once bound, angiogenin is endocytosed and translocated to the nucleus, thereby promoting the endothelial invasiveness necessary for blood vessel formation. Angiogenin induces vascularization of normal and malignant tissues, and abolishes protein synthesis by specifically hydrolyzing cellular tRNAs.

Alternative splicing results in two transcript variants encoding the same protein. This gene and the gene that encodes ribonuclease, RNase A family, 4 share promoters and 5' exons. Each gene splices to a unique downstream exon that contains its complete coding region.