Mef2 Proteins, Required for Muscle Differentiation, Bind an Essential Site in the Ig λ Enhancer

Abstract

Abstract The Ig λ light chain gene enhancer has two unique essential motifs, λA and λB. The transcription factors that bind the λB motif have been identified as Pu.1 and Pu.1-interacting partner (Pip). We report here that the λA site includes a binding site for the myocyte-specific enhancer factor 2 (Mef2) family of transcription factors. Mef2 proteins were first described in muscle cells and, in vertebrates, include four known members designated A to D. Using a λA electrophoretic-mobility shift assay (EMSA), in conjunction with a high affinity Mef2 binding site and anti-Mef2 Abs, we show that members of the Mef2 family are present in nuclear extracts of λ-producing B cells and bind the λA site. Functional assays using the chloramphenicol acetyltransferase (CAT) reporter construct containing three copies of the λA motif demonstrate that the λA sequence can function as an enhancer in conjunction with the thymidine kinase (TK) promoter and is regulated by Mef2 proteins. Extrapolating from other systems where transcriptional regulation by Mef2 has been studied, other transcription factors may be involved along with Mef2 in transcriptional regulation at the λA site.