LEM-like domain of lamina-associated polypeptide 2 (LAP2) and similar proteins
LAP2, also termed thymopoietin (TP), or thymopoietin-related peptide (TPRP), is composed of isoform alpha and isoforms beta/gamma and may be involved in chromatin organization and postmitotic reassembly. Some of the LAP2 isoforms are inner nuclear membrane proteins that can bind to nuclear lamins and chromatin, while others are nonmembrane nuclear polypeptides. All LAP2 isoforms contain an N-terminal lamina-associated polypeptide-Emerin-MAN1 (LEM)-domain that is connected to a highly divergent LEM-like domain by an unstructured linker. Both LEM and LEM-like domains share the same structural fold, mainly composed of two large parallel alpha helices. However, their biochemical nature of the solvent-accessible residues is completely different, which indicates the two domains may target different protein surfaces. The LEM domain is responsible for the interaction with the nonspecific DNA binding protein barrier-to-autointegration factor (BAF), and the LEM-like domain is involved in chromosome binding. The family also includes the yeast helix-extension-helix domain-containing proteins, Heh1p (formerly called Src1p) and Heh2p, and their uncharacterized homologs found mainly in fungi and several in bacteria. Heh1p and Heh2p are inner nuclear membrane proteins that might interact with nuclear pore complexes (NPCs). Heh1p is involved in mitosis. It functions at the interface between subtelomeric gene expression and transcription export (TREX)-dependent messenger RNA export through NPCs. The function of Heh2p remains ill-defined. Both Heh1p and Heh2p contain a LEM-like domain (also termed HeH domain), but lack a LEM domain.