The study of the various protein kinases reveals that, despite their considerably diversity, they have evolved from a common origin. Eleven conserved subdomains have been described that encompass the catalytic core of these enzymes. One of these conserved regions, subdomain II, contains an invariant lysine residue present in all known protein kinase catalytic domains. Two facts have suggested that this conserved lysine of subdomain II is essential for binding ATP: (i) several investigators have demonstrated that this residue is physically proximal to the ATP molecule, and (ii) conservative substitutions at this site render the kinase inactive. However, these results are also consistent with a functional role of the conserved lysine of subdomain II in orienting or facilitating the transfer of phosphate. To study in more detail the role of subdomain II, we have generated mutants of the protein-tyrosine kinase pp56lck that have single amino acid substitutions within the area surrounding the conserved residue Lys-273 in subdomain II. When compared with wild-type pp56lck, these mutants displayed profound reductions in their phosphotransfer efficiencies and small differences in their affinities for ATP. Further, the substitution of arginine for Lys-273 resulted in a mutant protein unable to transfer the gamma-phosphate of ATP but able to bind 8-azido-ATP with an efficiency similar to that of wild-type pp56lck. These results suggest that the region including Lys-273 of subdomain II is involved in the enzymatic process of phosphate transfer, rather than in anchoring ATP.