REPLACING A SURFACE LOOP ENDOWS RIBONUCLEASE-A WITH ANGIOGENIC ACTIVITY

Angiogenin (ANG) promotes the formation of blood vessels in animals. T his 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. RN ase is not angiogenic. A hybrid protein was produced in which 13 resid ues of a divergent surface loop of ANG were substituted for the analog ous 15 residues of RNase. The value of k(cat)/K-m for the cleavage of uridylyl(3'-->5')adenosine by this hybrid protein was 20-fold less tha n that of RNase but 10(5)-fold greater than that of ANG. The thermal s tability of the hybrid protein was also less than that of RNase. Never theless, the RNase/ANG hybrid protein promotes angiogenesis in mice at least as extensively as does authentic ANG. Thus we present a protein endowed with st noncognate biological activity simply by replacing a single element of secondary structure. In addition, a 13 residue pepti de corresponding to the surface loop of ANG inhibits endogenous angiog enesis in mice. These results support a model in which both a surface loop and a catalytic site are necessary for the promotion of blood ves sel formation by ANG or RNase. The dissection of structure/function el ements in ANG reveals a unique opportunity to develop new molecules th at modulate neovascularization.