Characterization of two isoforms of the skeletal muscle LIM protein 1, SLIM1. Localization of SLIM1 at focal adhesions and the isoform slimmer in the nucleus of myoblasts and cytoplasm of myotubes suggests distinct roles in the cytoskeleton and in nuclear-cytoplasmic communication.
We have cloned and characterized a novel isoform of the skeletal muscle LIM protein 1 (SLIM1), designated SLIMMER. SLIM1 contains an N-terminal single zinc finger followed by four LIM domains. SLIMMER is identical to SLIM1 over the first three LIM domains but contains a novel C-terminal 96 amino acids with three potential bipartite nuclear localization signals, a putative nuclear export sequence, and 27 amino acids identical to the RBP-J binding region of KyoT2, a murine isoform of SLIM1. SLIM1 localized to the cytosol of Sol8 myoblasts and myotubes. SLIMMER was detected in the nucleus of myoblasts and, following differentiation into myotubes, was exclusively cytosolic. Recombinant green fluorescent protein-SLIM1 localized to the cytoplasm and associated with focal adhesions and actin filaments in COS-7 cells, while green fluorescent protein-SLIMMER was predominantly nuclear. SLIMMER truncation mutants revealed that the first nuclear localization signal mediates nuclear localization. The addition of the proposed nuclear export sequence decreased the level of exclusively nuclear expression and increased cytosolic SLIMMER expression in COS-7 cells. The leucine-rich nuclear export signal was required for the export of SLIMMER from the nucleus of myoblasts to the cytoplasm of myotubes. Collectively, these results suggest distinct roles for SLIM1 and SLIMMER in focal adhesions and nuclear-cytoplasmic communication.
Brown
S
S
Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria, Australia 3168.
McGrath
M J
MJ
Ooms
L M
LM
Gurung
R
R
Maimone
M M
MM
Mitchell
C A
CA
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001854:Bone Marrow Cells / Q000737:chemistry; D019556:COS Cells; D002448:Cell Adhesion; D002467:Cell Nucleus / Q000378:metabolism; D003001:Cloning, Molecular; D003593:Cytoplasm / Q000378:metabolism; D003599:Cytoskeleton / Q000378:metabolism; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D060588:LIM Domain Proteins; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000235:genetics / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000737:chemistry*; D014162:Transfection; D016335:Zinc Fingers
C101149:FHL1 protein, human; D047908:Intracellular Signaling Peptides and Proteins; D060588:LIM Domain Proteins; D009124:Muscle Proteins