Histidine Residue 102 and 117 of MELC1 Play Different Roles in the Chaperone Function for Streptomyces Apotyrosinase

MelC1 regulates the copper incorporation and secretion of Streptomyces apotyrosinase (MelC2) via a transient, competent complex formation. His-102 and His-117 of the chaperone-like MelC1 are known to play important roles in this trans-activation activity of MelC1. In this study, we studied the side chain requirement at these two residues for MelC1 function. Substitution of His-117 with polar, charged, or hydrophobic amino acid resulted in complete abolishment of the tyrosinase activity but no effect on its secretion. When similar amino acid substitutions were introduced at His-102, both tile secretion and the enzymatic activity of tyrosinase were blocked to different extents. Furthermore, the tyrosinase activity of the His-117 mutants but not the His-102 mutants could be partially reactivated by in vitro copper reconstitution. Notably, the defects in the MelC1 mutant protein did not impair the formation of the MelC1·MelC2 binary complex, but rather produced an incompetent complex. In summary, our results reveal that His-102 and His-117 of MelC1 play different roles in MelC1 functions. In particular, His-117 of MelC1 plays a pivotal role in regulation of the copper incorporation but not the secretion of apotyrosinase, while His-102 plays a dual role in both the secretion and the activation of apotyrosinase.