Evolution and function of leukocyte RNase A ribonucleases of the avian species, Gallus gallus.
In this study, we explore the evolution and function of two closely related RNase A ribonucleases from the chicken, Gallus gallus. Separated by approximately 10 kb on chromosome 6, the coding sequences of RNases A-1 and A-2 are diverging under positive selection pressure (dN > dS) but remain similar to one another (81% amino acid identity) and to the mammalian angiogenins. Immunoreactive RNases A-1 and A-2 (both approximately 16 kDa) were detected in peripheral blood granulocytes and bone marrow. Recombinant proteins are ribonucleolytically active (kcat = 2.6 and 0.056 s(-1), respectively), and surprisingly, both interact with human placental ribonuclease inhibitor. RNase A-2, the more cationic (pI 11.0), is both angiogenic and bactericidal; RNase A-1 (pI 10.2) has neither activity. We demonstrated via point mutation of the catalytic His110 that ablation of ribonuclease activity has no impact on the bactericidal activity of RNase A-2. We determined that the divergent domains II (amino acids 71-76) and III (amino acids 89-104) of RNase A-2 are both important for bactericidal activity. Furthermore, we demonstrated that these cationic domains can function as independent bactericidal peptides without the tertiary structure imposed by the RNase A backbone. These results suggest that ribonucleolytic activity may not be a crucial constraint limiting the ongoing evolution of this gene family and that the ribonuclease backbone may be merely serving as a scaffold to support the evolution of novel, nonribonucleolytic proteins.
Nitto
Takeaki
T
Laboratory of Allergic Diseases and Research Technologies Branch, NIAID, National Institutes of Health, Bethesda, Maryland 20892, USA.
Dyer
Kimberly D
KD
Czapiga
Meggan
M
Rosenberg
Helene F
HF
D000595:Amino Acid Sequence; D000818:Animals; D001854:Bone Marrow Cells / Q000378:metabolism; D002645:Chickens; D019143:Evolution, Molecular; D006801:Humans; D007700:Kinetics; D007962:Leukocytes / Q000201:enzymology* / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010802:Phylogeny; D017434:Protein Structure, Tertiary; D012259:Ribonuclease, Pancreatic / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid
D012259:Ribonuclease, Pancreatic