Functional and structural role of arginine 103 in human erythropoietin

The identification of amino acid residues essential for function of the hematopoietic growth factor erythropoietin has been approached by several methods, including comparisons of related sequences, immunochemical approaches, mutagenesis, and computer modeling. We have reported previously that mutations within amino acids 100-109 of erythropoietin can have profound effects on the hormone's structure and/or activity and that Arg103 is especially important for function [Y. Chern, T. Chung, and A. J. Sytkowski (1991) Eur. J. Biochem. 202, 225-229; J. Grodberg, K.L. Davis, and A. J. Sytkowski (1993) Eur. J. Biochem. 218, 597-601]. We have now constructed a series of Arg103 substitutions in order to determine the structural features of amino acid 103 required for biological activity. Each of the mutants was expressed and secreted efficiently by transfected COS1 cells. Mutants Arg103Asn, Arg103Gln, and Arg103Glu exhibited no biological activity. In contrast, Arg103His and Arg103Lys had specific activities equal to 2 and 25%, respectively, of that of wild-type erythropoietin, indicating that a positive charge may be required at position 103 but that other constraints necessitate the presence of Arg for full activity. A role for amino acid 103 in the protein's structure was supported by the results of experiments which revealed marked differences in heat stability among the mutants. We hypothesize that an Arg at position 103 may confer sufficient flexibility to the receptor binding domain to facilitate initial binding to the receptor and may then stabilize the binary complex by hydrogen bonding with carbonyls of the receptor protein.