Direct interaction of tyrosinase with Tyrp1 to form heterodimeric complexes in vivo

Mutations of the critical and rate-limiting melanogenic enzyme tyrosinase (Tyr) result in hypopigmentation of the hair, skin and eyes. Two other related enzymes, Tyrp1 and Dct, catalyze distinct post-Tyr reactions in melanin biosynthesis. Tyr, Tyrp1 and Dct have been proposed to interact with and stabilize each other in multi-enzyme complexes, and in vitro, Tyr activity is more stable in the presence of Tyrp1 and/or Dct. We recently reported that Tyr is degraded more quickly in mutant Tyrp1 mouse melanocytes than in wild-type Tyrp1 melanocytes, and that decreased stability of Tyr can be partly rescued by infection with wild-type Tyrp1. Although interactions between Tyr and Tyrp1 have been demonstrated in vitro, there is no direct evidence for Tyr interaction with Tyrp1 in vivo. In this study, we use in vivo chemical crosslinking to stabilize the association of Tyr with other cellular proteins. Western blot analysis revealed that Tyrp1, but not Dct, associates with Tyr in murine melanocytes in vivo, and more specifically, in melanosomes. Two-dimensional SDS-PAGE analysis detected heterodimeric species of Tyr and Tyrp1. Taken together, these data demonstrate that Tyrp1 interacts directly with Tyr in vivo, which may regulate the stability and trafficking of melanogenic enzymes and thus pigment synthesis.