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.

X-linked Emery-Dreifuss muscular dystrophy is caused by loss of emerin, a LEM-domain protein of the nuclear inner membrane. To better understand emerin function, we used affinity chromatography to purify emerin-binding proteins from nuclear extracts of HeLa cells. Complexes that included actin, αII-spectrin and additional proteins, bound specifically to emerin. Actin polymerization assays in the presence or absence of gelsolin or capping protein showed that emerin binds and stabilizes the pointed end of actin filaments, increasing the actin polymerization rate 4- to 12-fold. We propose that emerin contributes to the formation of an actin-based cortical network at the nuclear inner membrane, conceptually analogous to the actin cortical network at the plasma membrane. Thus, in addition to disrupting transcription factors that bind emerin, loss of emerin may destabilize nuclear envelope architecture by weakening a nuclear actin network.

Emerin, a conserved LEM-domain protein, is among the few nuclear membrane proteins for which extensive basic knowledge—biochemistry, partners, functions, localizations, posttranslational regulation...

LAP2 belongs to a family of nuclear membrane proteins sharing a 43 residue LEM domain. All LAP2 isoforms have the same N‐terminal ‘constant’ region (LAP2‐c), which includes the LEM domain, plus a C‐terminal ‘variable’ region. LAP2‐c polypeptide inhibits nuclear assembly in Xenopus extracts, and binds in vitro to barrier‐to‐autointegration factor (BAF), a DNA‐bridging protein. We tested 17 Xenopus LAP2‐c mutants for nuclear assembly inhibition, and binding to BAF and BAF·DNA complexes. LEM domain mutations disrupted all activities tested. Some mutations outside the LEM domain had no effect on binding to BAF, but disrupted activity in Xenopus extracts, suggesting that LAP2‐c has an additional unknown function required to inhibit nuclear assembly. Mutagenesis results suggest that BAF changes conformation when complexed with DNA. The binding affinity of LAP2 was higher for BAF·DNA complexes than for BAF, suggesting that these interactions are physiologically relevant. Nucleoplasmic domains of Xenopus LAP2 isoforms varied 9‐fold in their affinities for BAF, but all isoforms supershifted BAF·DNA complexes. We propose that the LEM domain is a core BAF‐binding domain that can be modulated by the variable regions of LAP2 isoforms.