Characterization of the chicken L-Maf, MafB and c-Maf in crystallin gene regulation and lens differentiation

Background: Members of the Maf family, including L-Maf, MafB and c-Maf, are 'basic region/leucine zipper' (bZIP) transcription factors. Maf proteins contain a highly conserved acidic transactivation domain (AD), and a bZIP region that mediates DNA-binding activity. The hinge region between AD and bZIP varies considerably in length between different proteins. Recent studies reveal that L-Maf, c-Maf and MafB play key roles in vertebrate lens development. Results: We investigated the transactivation activity of individual factors in culture cells to analyse their specific functions. In transient transfection assays with a reporter gene containing Maf responsive elements, MafB and c-Maf activated higher levels of the reporter gene than L-Maf. However, L-Maf transactivated the alphaA-crystallin promoter as effectively as MafB and c-Maf, and induced the expression of the endogenous delta-crystallin gene more efficiently than the other two proteins. Domain-swapping experiments reveal that the bZIP region of MafB takes part in strong transcriptional activity, while the acidic and hinge regions (AH) of c-Maf collectively serve as a strong transactivation domain. The AH region of L-Maf (but not c-Maf) conferred transactivation activity to induce delta-crystallin gene expression. Conclusions: These results suggest that despite their similar DNA binding properties, L-Maf, MafB and c-Maf regulate different sets of target genes by complex interactions with multiple factors that recognize cis -elements in promoters. The AH region of L-Maf has a distinct role in inducing endogenous delta-crystallin gene.