TRANSCRIPTIONAL SYNERGY BETWEEN LIM-HOMEODOMAIN PROTEINS AND BASIC HELIX-LOOP-HELIX PROTEINS - THE LIM2 DOMAIN DETERMINES SPECIFICITY

LIM-homeodomain proteins direct cellular differentiation by activating transcription of cell-type-specific genes, but this activation requir es cooperation with other nuclear factors. The LIM-homeodomain protein Lmx1 cooperates with the basic helix-loop-helix (bHLH) protein E47/Pa n-1 to activate the insulin promoter in transfected fibroblasts. In th is study, we show that two proteins originally called Lmx1 are the clo sely related products of two distinct vertebrate genes, Lmx1.1 and Lmx 1.2, We have used yeast genetic systems to delineate the functional do mains of the Lmx1 proteins and to characterize the physical interactio ns between Lmx1 proteins and E47/Pan-1 that produce synergistic transc riptional activation. The LIM domains of the Lmx1 proteins, and partic ularly the second LIM domain, mediate both specific physical interacti ons and transcriptional synergy with E47/Pan-1. The LIM domains of the LIM-homeodomain protein Isl-1, which cannot mediate transcriptional s ynergy with E47/Pan-1, do not interact with E47/Pan-1. In vitro studie s demonstrate that the Lmx1.1 LIM2 domain interacts specifically with the bHLH domain of E47/Pan-1, These studies provide the basis for a mo del of the assembly of LIM-homeodomain-containing completes on DNA ele ments that direct cell-type-restricted transcription in differentiated tissues.