Enzymically active angiogenin/ribonuclease A hybrids formed by peptide interchange

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Abstract
The primary structures of the blood vessel inducing protein human angiogenin and human pancreatic ribonuclease (RNase) are 35% identical. Angiogenin catalyses the limited cleavage of ribosomal RNA (18 and 28 S), yielding a characteristic pattern of polynucleotide products, but shows no significant activity toward conventional pancreatic RNase substrates [Shapiro, R., Riordan, J.F., and Vallee, B.L., (1986) Biochemistry 25, 3527-3532]. Angiogenin/RNase hybrid enzymes-wherein particular regions of primary structure in RNase are replaced by the corresponding segments of angiogenin-serve to explore the structural features underlying angiogenin''s characteristic activities. Herein we show that synthetic angiogenin peptides, Ang(1-21) and Ang(108-123), form noncovalent complexes with inactive fragments of bovine RNase A-RNase(21-124) (i.e., S-protein) and RNase(1-118), respectively-with regeneration of activity toward conventional RNase substrates. Maximal activities for the Ang(1-21)/S-protein complex (Kd = 1.0 .mu.M) are 52%, and 15% toward cytidine cyclic 2'',3''-phosphate, cytidylyl(3''.fwdarw.5'')adenosine, and yeast RNA, respectively. In contrast, activities of RNase(1-118)/Ang(108-123) hybrid (Kd = 25 .mu.M) are 1-2 orders of magnitude lower toward cyclic nucleotides and dinucleoside phosphates. However, substitution of phenylalanine for Leu-115 in Ang(108-123) increases activity up to 100-fold. Both His-13 and His-114 in the angiogenin peptides are required for activity since their substitution by alanine yields inactive complexes. Importantly, the pattern of polynucleotide products formed during cleavage of ribosomal RNA by the Ang(1-21)/S-protein hybrid shows a striking resemblance to that formed by angiogenin, demonstrating that the hybrid retains features of both angiogenin and RNase A. In contrast, neither RNase(1-118)/Ang(108-123) nor S-peptide/S-protein complexes produce this cleavage pattern. Thus, the data point to an important role for the N-terminal region of angiogenin in conferring substrate selectivity.