Meis proteins are major in vivo DNA binding partners for wild-type but not chimeric Pbx proteins.

The Pbx1 and Meis1 proto-oncogenes code for divergent homeodomain proteins that are targets for oncogenic mutations in human and murine leukemias, respectively, and implicated by genetic analyses to functionally collaborate with Hox proteins during embryonic development and/or oncogenesis. Although Pbx proteins have been shown to dimerize with Hox proteins and modulate their DNA binding properties in vitro, the biochemical compositions of endogenous Pbx-containing complexes have not been determined. In the present study, we demonstrate that Pbx and Meis proteins form abundant complexes that comprise a major Pbx-containing DNA binding activity in nuclear extracts of cultured cells and mouse embryos. Pbx1 and Meis1 dimerize in solution and cooperatively bind bipartite DNA sequences consisting of directly adjacent Pbx and Meis half sites. Pbx1-Meis1 heterodimers display distinctive DNA binding specificities and cross-bind to a subset of Pbx-Hox sites, including those previously implicated as response elements for the execution of Pbx-dependent Hox programs in vivo. Chimeric oncoprotein E2a-Pbx1 is unable to bind DNA with Meis1, due to the deletion of amino-terminal Pbx1 sequences following fusion with E2a. We conclude that Meis proteins are preferred in vivo DNA binding partners for wild-type Pbx1, a relationship that is circumvented by its oncogenic counterpart E2a-Pbx1.     

MyoD targets chromatin remodeling complexes to the myogenin locus prior to forming a stable DNA-bound complex

The activation of muscle-specific gene expression requires the coordinated action of muscle regulatory proteins and chromatin-remodeling enzymes. Microarray analysis performed in the presence or absence of a dominant-negative BRG1 ATPase demonstrated that approximately one-third of MyoD-induced genes were highly dependent on SWI/SNF enzymes. To understand the mechanism of activation, we performed chromatin immunoprecipitations analyzing the myogenin promoter. We found that H4 hyperacetylation preceded Brg1 binding in a MyoD-dependent manner but that MyoD binding occurred subsequent to H4 modification and Brg1 interaction. In the absence of functional SWI/SNF enzymes, muscle regulatory proteins did not bind to the myogenin promoter, thereby providing evidence for SWI/SNF-dependent activator binding. We observed that the homeodomain factor Pbx1, which cooperates with MyoD to stimulate myogenin expression, is constitutively bound to the myogenin promoter in a SWI/SNF-independent manner, suggesting a two-step mechanism in which MyoD initially interacts indirectly with the myogenin promoter and attracts chromatin-remodeling enzymes, which then facilitate direct binding by MyoD and other regulatory proteins.     

Transcriptional analysis of the titin cap gene

Mutations in titin cap (Tcap), also known as telethonin, cause limb-girdle muscular dystrophy type 2G (LGMD2G). Tcap is one of the titin interacting Z-disc proteins involved in the regulation and development of normal sarcomeric structure. Given the essential role of Tcap in establishing and maintaining normal skeletal muscle architecture, we were interested in determining the regulatory elements required for expression of this gene in myoblasts. We have defined a highly conserved 421 bp promoter proximal promoter fragment that contains two E boxes and multiple putative Mef2 binding sequences. This promoter can be activated by MyoD and myogenin in NIH3T3 fibroblast cells, and maintains the differentiated cell-specific expression pattern of the endogenous Tcap in C2C12 cells. We find that while both E boxes are required for full activation by MyoD or myogenin in NIH3T3 cells, the promoter proximal E box has a greater contribution to activation of this promoter in C2C12 cells and to activation by MyoD in NIH3T3 cells. Together, the data suggest an important role for MyoD in activating Tcap expression through the promoter proximal E box. We also show that myogenin is required for normal expression in vivo and physically binds to the Tcap promoter during embryogenesis.