Replacing a Surface Loop Endows Ribonuclease A with Angiogenic Activity

Angiogenin (ANG) promotes the formation of blood vessels in animals. This hormone is a small, monomeric protein that is homologous to bovine pancreatic ribonuclease A (RNase). ANG is a poor ribonuclease but its ribonucleolytic activity is essential for its angiogenic activity. RNase is not angiogenic. A hybrid protein was produced in which 13 residues of a divergent surface loop of ANG were substituted for the analogous 15 residues of RNase. The value of kcat/Km for the cleavage of uridylyl(3'-->5')adenosine by this hybrid protein was 20-fold less than that of RNase but 10(5)-fold greater than that of ANG. The thermal stability of the hybrid protein was also less than that of RNase. Nevertheless, the RNase/ANG hybrid protein promotes angiogenesis in mice at least as extensively as does authentic ANG. Thus we present a protein endowed with a noncognate biological activity simply by replacing a single element of secondary structure. In addition, a 13-residue peptide corresponding to the surface loop of ANG inhibits endogenous angiogenesis in mice. These results support a model in which both a surface loop and a catalytic site are necessary for the promotion of blood vessel formation by ANG or RNase. The dissection of structure/function elements in ANG reveals a unique opportunity to develop new molecules that modulate neovascularization.