Distinct functional domains in emerin bind lamin A and DNA-bridging protein BAF

Abstract

Loss of emerin, a lamin-binding nuclear membrane protein, causes Emery-Dreifuss muscular dystrophy. We analyzed 13 site-directed mutations, and four disease-causing mutations that do not disrupt emerin stability or localization. We show that emerin binds directly to barrier-to-autointegration factor (BAF), a DNA-bridging protein, and that this binding to BAF requires conserved residues in the LEM-motif of emerin. Emerin has two distinct functional domains: the LEM-domain at the N-terminus, which mediates binding to BAF, and a second functional domain in the central region, which mediates binding to lamin A. Disease mutation Δ95-99 mapped to the lamin-binding domain and disrupted lamin A binding in vitro. Two other disease-linked residues, Ser54 and Pro183, mapped outside the BAF and lamin-binding domains, suggesting that emerin may have additional functional domains relevant to disease. The disease-linked emerin proteins all remained active for binding to BAF, both in vitro and in vivo, suggesting that disease can result from the loss of specific molecular interactions between emerin and either lamin A or putative novel partner(s). The demonstration that emerin binds directly to BAF, coupled to similar results for LAP2, provides proof in principle that all LEM-domain nuclear proteins can interact with BAF, with interesting implications for chromatin attachment to the nuclear envelope.

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“…Thus, BAF is a key NE localization factor for both emerin and lamin A post-mitotically [52]. Together, these finding indicate that the recruitment and retention of emerin may involve sequential interactions with BAF and lamins [52].…”

“…Recent studies have also suggested that BAF may be critically important for emerin NE localization, as emerin and lamin A both fail to associate with assembling nuclear envelope in cells that express a dominant mutant of BAF [52]. Thus, BAF is a key NE localization factor for both emerin and lamin A post-mitotically [52]. Together, these finding indicate that the recruitment and retention of emerin may involve sequential interactions with BAF and lamins [52].…”

“…Emerin residues 70-178 bind to lamin A [50] and mediate its NE enrichment [51]. Recent studies have also suggested that BAF may be critically important for emerin NE localization, as emerin and lamin A both fail to associate with assembling nuclear envelope in cells that express a dominant mutant of BAF [52]. Thus, BAF is a key NE localization factor for both emerin and lamin A post-mitotically [52].…”

“…To map emerin functional domains, clustered Ala-substitution mutations have been generated in recombinant emerin. One set targets residues, that are identical between emerin and LAP2β, which is thought to mediate shared functions [52]. Another set of mutations targets residues that differ between emerin and LAP2β; these are predicted to disrupt emerin-specific functions [59].…”

“…Emerin binds a number of transcription regulators, including GCL [59], Btf [62], Lmo7 [63], ß-catenin [64], SIKE [65], and BAF [52], and regulates the expression of their target genes. Emerin has been shown to regulate the expression of many muscles and cardiac genes [44,66,67].…”