The lymphocyte-specific protein LSP1 binds to F-actin and to the cytoskeleton through its COOH-terminal basic domain.
The lymphocyte-specific phosphoprotein LSP1 associates with the cytoplasmic face of the plasma membrane and with the cytoskeleton. Mouse LSP1 protein contains 330 amino acids and contains an NH2-terminal acidic domain of approximately 177 amino acids. The COOH-terminal half of the LSP1 protein is rich in basic residues. In this paper we show that LSP1 protein which is immunoprecipitated with anti-LSP1 antibodies from NP-40-soluble lysates of the mouse B-lymphoma cell line BAL17 is associated with actin. In vitro binding experiments using recombinant LSP1 (rLSP1) protein and rabbit skeletal muscle actin show that LSP1 binds along the sides of F-actin but does not bind to G-actin. rLSP1 does not alter the initial polymerization kinetics of actin. The highly conserved COOH-terminal basic domains of mouse and human LSP1 share a significant homology with the 20-kD COOH-terminal F-actin binding fragment of caldesmon. A truncated rLSP1 protein containing the entire COOH-terminal basic domain from residue 179 to 330, but not the NH2-terminal acidic domain binds to F-actin at least as well as rLSP1. When LSP1/CAT fusion proteins are expressed in a LSP1-negative T-lymphoma cell line, only fusion proteins containing the basic COOH-terminal domain associate with the NP-40-insoluble cytoskeleton. These data show that LSP1 binds F-actin through its COOH-terminal basic domain and strongly suggest that LSP1 interacts with the cytoskeleton by direct binding to F-actin. We propose that LSP1 plays a role in mediating cytoskeleton driven responses in lymphocytes such as receptor capping, cell motility, or cell-cell interactions.
Jongstra-Bilen
J
J
Department of Immunology, University of Toronto, Ontario, Canada.
Janmey
P A
PA
Hartwig
J H
JH
Galea
S
S
Jongstra
J
J
D000199:Actins / Q000378:metabolism* / Q000648:ultrastructure; D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D003599:Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D005455:Fluorescent Antibody Technique; D007700:Kinetics; D051379:Mice; D008840:Microfilament Proteins; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D011817:Rabbits; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D014407:Tumor Cells, Cultured
D000199:Actins; D002135:Calcium-Binding Proteins; C489183:LSP1 protein, human; C057701:Lsp1 protein, mouse; D008840:Microfilament Proteins; D010750:Phosphoproteins; D011994:Recombinant Proteins