Precise regulation of basic Helix-Loop-Helix (bHLH) gene activity is crucial for normal development. Aberrant activity and expression of bHLH genes has been implicated in many diseases, emphasizing the importance of understanding the mechanisms regulating bHLH genes. Previous work revealed that HLH-2, the ortholog of mammalian E2A, is regulated post-transcriptionally during C. elegans ventral uterine development (Karp and Greenwald 2003). Here we investigate this finding further and identify a novel mechanism for regulating bHLH genes: dimerization-driven protein degradation.
In the somatic gonad, hlh-2 has distinct, sequential roles in the specification and function of the anchor cell (AC). The AC is the specialized cell in the ventral uterus that serves as a signaling nexus for uterine and vulval patterning. Four cells initially have the potential to be the AC; only one becomes the AC, and the other three become ventral uterine precursor cells (VUs). hlh-2 plays multiple and sequential roles in AC specification and differentiation: hlh-2 endows prospective uterine cells with the potential to be the AC; resolves which cell becomes the AC in a cell-cell interaction mediated by LIN-12/Notch; and promotes differentiation and function of the AC by directly activating transcription of target genes that organize uterine and vulval development.
HLH-2 is an obligate dimer, and can homodimerize or heterodimerize with other bHLH proteins. We found that HLH-2 homodimers carry out each of these three functions in the AC. To determine what features of the HLH-2 homodimer mediate its differential stability in the AC and VUs, we first identified an element from the hlh-2 5' flanking region, “hlh-2prox,” that drives expression in the four ventral uterine cells. We then expressed GFP-tagged mutant forms of HLH-2 and assayed the effect on GFP-HLH-2 stability and function. Our analysis revealed that a functional dimerization domain of HLH-2 is necessary and sufficient for downregulation in VUs. By ectopically expressing other bHLH proteins in uterine cells, we also found that dimerization-driven downregulation can recognize and target other bHLH dimers as well. Remarkably, we also found that mammalian E2A isoforms can functionally substitute for C. elegans HLH-2 in regulating AC development, and furthermore, that E2A isoforms also display dimerization-dependent degradation in VUs. Our results suggest that dimerization-driven regulation of bHLH protein stability may be a conserved mechanism for differential regulation of these ancient proteins in specific cell contexts.
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