MAN1, an Inner Nuclear Membrane Protein That Shares the LEM Domain with Lamina-associated Polypeptide 2 and Emerin

Abstract

The "MAN antigens" are polypeptides recognized by autoantibodies from a patient with a collagen vascular disease and localized to the nuclear envelope. We now show that one of the human MAN antigens termed MAN1 is a 82.3-kDa protein with an amino-terminal domain followed by two hydrophobic segments and a carboxylterminal tail. The MAN1 gene contains seven proteincoding exons and is assigned to human chromosome 12q14. Its mRNA is approximately 5.5 kilobases and is detected in several different cell types that were examined. Cell extraction experiments show that MAN1 is an integral membrane protein. When expressed in transfected cells, MAN1 is exclusively targeted to the nuclear envelope, consistent with an inner nuclear membrane localization. Protein sequence analysis reveals that MAN1 shares a conserved globular domain of approximately 40 amino acids, which we term the LEM module, with inner nuclear membrane proteins lamina-associated polypeptide 2 and emerin. The LEM module is also present in two proteins of Caenorhabditis elegans. These results show that MAN1 is an integral protein of the inner nuclear membrane that shares the LEM module with other proteins of this subcellular localization.Only a few proteins other than the nuclear lamins, some with various isoforms, have been localized to the inner nuclear membrane during interphase. The first to be identified was avian lamin B receptor (LBR) 1 (1, 2). LBR was subsequently characterized in mammals (3-6), and an immunochemically crossreactive protein has been identified in sea urchins (7). LBR has a nucleoplasmic, amino-terminal domain of approximately 200 amino acids that binds to B-type lamins and chromatin proteins and confers inner nuclear membrane retention (2, 4, 8 -12). The amino-terminal domain of LBR is followed by a hydrophobic domain with eight putative transmembrane segments that is similar in sequence to sterol reductases, including two human proteins of the endoplasmic reticulum, one of which is a 7-dehydrocholesterol reductase (13).Two other integral proteins of the inner nuclear membrane have been termed lamina-associated polypeptides (LAPs). Three related isoforms of rat LAP1 (LAP1A, LAP1B, and LAP1C) were identified by reaction with a single monoclonal antibody and shown to be integral membrane proteins associated with the nuclear lamina (14). LAP1C has a nucleoplasmic amino-terminal domain followed by one transmembrane segment, and the other LAP1 isoforms are probably of similar overall structure, arising from the same gene by alternative RNA splicing (15). LAP2 was also first identified by reaction with a monoclonal antibody and shown to be an integral membrane protein that binds to nuclear lamins and chromatin (16). Several isoforms of LAP2, which have also been called thymopoietins because they were thought to possibly be thymocyte growth factors, are generated by alternative RNA splicing (17-21). Some of the LAP2 isoforms are integral proteins of the inner nuclear membrane, with nucleoplasmic amino-terminal domains and single transmembr...

Inner nuclear membrane and signal transduction

“…The MAN1 antigen with an approximate molecular mass of 60 kDa was subsequently identified as laminaassociated polypeptide 2-beta [Lang et al, 1999]. Using ''MAN antiserum'' to screen expression libraries, a partial cDNA for the largest recognized antigen was obtained and isolation of overlapping cDNAs led to the characterization of MAN1 [Lin et al, 2000].…”

“…The amino-terminal domain preceding the first transmembrane segment contains approximately 450 amino acids and faces the nucleoplasm. The first approximately 50 amino acids of MAN1 is a LEM domain, a helix-loop-helix motif found in lamina-associated polypeptide 2, emerin, MAN1 and several other nuclear proteins [Lin et al, 2000;Cai et al, 2001;Laguri et al, 2001]. The LEM domain has been shown to bind to barrier-to-autointegration factor [Cai et al, 2001].…”

“…The carboxyl-terminal domain following the second transmembrane segment of MAN1 contains approximately 230 amino acids and faces the nucleoplasm. The human gene encoding MAN1 was localized to chromosome 12q14 [Lin et al, 2000].…”

The interaction of two classes of nuclear vesicles is induced by the dissociation of soluble proteins from one class of vesicles

“…The outer nuclear membrane is continuous with the endoplasmic reticulum (Franke, 1974), but the inner nuclear membrane associates with the nuclear lamina, a meshwork of intermediate filaments (Aebi et al, 1988), on its nucleoplasmic side. The inner nuclear membrane contains a unique set of membrane proteins, lamin B receptor (LBR) (Worman et al, 1988), LAPs (Foisner and Gerace, 1993), emerin (Manilal et al, 1996), otefin (Ashery et al, 1997), nurim (Rolls et al, 1999), and MAN1 (Lin et al, 2000), which are not present in the outer nuclear membrane or the endoplasmic reticulum.…”

Organelle Proteome Variation Among Different Cell Types: Lessons from Nuclear Membrane Proteins

“…Interestingly, genomics itself was inadequate for the identification of NETs. For example, a weak homology "LEM" domain was identified when MAN1 was cloned and sequenced (Lin et al, 2000) because this domain was also found to occur in LAP2 and emerin (hence LEM for LAP2, emerin, and MAN1). However, not all proteins containing LEM domains are concentrated at the NE (Dechat et al, 1998;Raju et al, 2003) and the presence of a LEM domain is not always maintained within a protein family e.g.…”