There are two genes encoding members of a new family of type II integral membrane proteins. Both are ubiquitously expressed, and tissue-specific alternative mRNA initiation and splicing generate at least two major isoforms of each protein, with the smaller isoforms being truncated at the N-terminus. These proteins are called nesprin-l and -2 for nuclear _envelope _spectrin repeat, as they are characterized by the presence of multiple, clustered spectrin repeats, bipartite nuclear localization sequences and a conserved C-terminal, single transmembrane domain. Transient transfection of EGFP-fusion expression constructs demonstrated their localization to the nuclear membrane with a novel C-terminal, TM-domain-containing sequence essential for perinuclear localization. Nesprin-l is developmentally regulated in both smooth and skeletal muscle and is relocalized from the nuclear envelope to the nucleus and cytoplasm during C2Cl2 myoblast differentiation. Nesprins may function as 'dystrophins of the nucleus' to maintain nuclear organization and structura integrity.
CTP synthase 1; CTP synthetase 1; cytidine 5'-triphosphate synthetase; cytidine 5-prime triphosphate synthetase; UTP--ammonia ligase 1