The microphthalmia transcription factor (MITF) contains two N-terminal domains required for transactivation of osteoclast target promoters and rescue of mi mutant osteoclasts.

The microphthalmia transcription factor (MITF) regulates gene expression during differentiation of several distinct cell types, including osteoclasts. A structure/function analysis was performed to determine whether transcription activation domains were important for MITF action in osteoclasts. In addition to a previously characterized acidic activation necessary for melanocyte differentiation, the analysis defined a second potential activation domain located between amino acids 140 and 185. This second domain is required for MITF transactivation of two probable targets, the E-cadherin promoter and the tartrate-resistant acid phosphatase promoter, in transient transfection assays. An intact MITF gene rescued differentiation when introduced into osteoclasts derived from mi/mi mice using a retrovirus vector. In parallel experiments, an MITF gene lacking the acidic-activation domain rescued differentiation twofold less efficiently than wild type, and a gene lacking the region between amino acid residues 140 and 185 rescued differentiation tenfold less efficiently than wild type. The results indicate that the N-terminal region of MITF is necessary for activation of gene expression in osteoclasts and provides one mechanism by which this factor regulates distinct target genes in different cell types.