Eosinophil cationic protein and eosinophil-derived neurotoxin. Evolution of novel function in a primate ribonuclease...

Human eosinophil-derived neurotoxin (EDN) and eosinophil cationic protein (ECP) are members of a unique subfamily of rapidly evolving primate ribonuclease genes that emerged via a gene duplication event occurring after the divergence of Old World from New World monkeys (Rosenberg, H. F., Dyer, K. D., Tiffany, H. L., and Gonzalez, M. (1995) Nature Genet. 10, 219-223). In this work, we studied the activity of the protein encoded by the EDN/ECP homolog of the New World monkey, Saguinus oedipus (marmoset), a representative of the "ancestral" single sequences. Although the nucleotide sequence of the single marmoset gene (mEDN) was equally homologous (82%) to both human genes, the encoded amino acid sequence, calculated isoelectric point, and immunoreactivity all suggested a closer relationship with EDN. Furthermore, mEDN (at 0.3-1.0 microM concentrations) had no measurable anti-staphylococcal activity, suggesting functional as well as structural similarity to EDN. However, with yeast tRNA as substrate, mEDN had significantly less ribonuclease activity than EDN; Michaelis constants were nearly identical (Km (mEDN) = 0.67 microM; Km (EDN) = 0.70 microM), while turnover numbers differed by a factor of 100 (kcat (mEDN) = 0.91 s-1; kcat (EDN) = 0.64 x 10(-2) s-1). Thus, evolutionary constraints appear to have promoted two novel functions: increased cationicity/toxicity (ECP) and enhanced ribonuclease activity (EDN). The latter result is particularly intriguing, as it suggests a crucial role for ribonuclease activity in the (as yet to be determined) physiologic function of EDN.