LEM2 is a novel MAN1-related inner nuclear membrane protein associated with A-type lamins

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Abstract
The LEM (lamina-associated polypeptide–emerin–MAN1) domain is a motif shared by a group of lamin-interacting proteins in the inner nuclear membrane (INM) and in the nucleoplasm. The LEM domain mediates binding to a DNA-crosslinking protein, barrier-to-autointegration factor (BAF). We describe a novel, ubiquitously expressed LEM domain protein, LEM2, which is structurally related to MAN1. LEM2 contains an N-terminal LEM motif, two predicted transmembrane domains and a MAN1-Src1p C-terminal (MSC) domain highly homologous to MAN1, but lacks the MAN1-specific C-terminal RNA-recognition motif. Immunofluorescence microscopy of digitonin-treated cells and subcellular fractionation identified LEM2 as a lamina-associated protein residing in the INM. LEM2 binds to the lamin C tail in vitro. Targeting of LEM2 to the nuclear envelope requires A-type lamins and is mediated by the N-terminal and transmembrane domains. Highly overexpressed LEM2 accumulates in patches at the nuclear envelope and forms membrane bridges between nuclei of adjacent cells. LEM2 structures recruit A-type lamins, emerin, MAN1 and BAF, whereas lamin B and lamin B receptor are excluded. Our data identify LEM2 as a novel A-type-lamin-associated INM protein involved in nuclear structure organization.