Conserved cysteine residues are critical for the enzymatic function of the lymphocyte-specific tyrosine protein kinase p56lck.

We have evaluated the possibility that conserved cysteine residues are critical for the enzymatic function of p56lck. Through oligonucleotide-directed mutagenesis, 5 Lck residues (cysteines 217, 224, 378, 464, and 475) were individually mutated to alanines, and the effects of these substitutions were tested in various in vitro and in vivo assays. We found that mutation of either of 2 cysteines located in the carboxyl portion of the kinase domain (cysteines 464 and 475) abolished the catalytic function of Lck. In addition, it was noted that alteration of cysteine 475 resulted in a dramatic reduction of the half-life of p56lck. These cysteine residues are highly conserved throughout the tyrosine protein kinase family, suggesting that they may play important functions in catalysis and/or substrate recognition.