LSP-1 (Lymphocyte Specific Protein-1) is expressed in lymphocytes (B cells, cytotoxic T cells, and helper T cells), neutrophils, and macrophages and is involved in lymphocyte signal transduction. LSP-1 is a cytoplasmic membrane-bound protein that, following activation, translocates to the cytosol. This activation and translocation is mediated by phosphorylation of serine and threonine residues in the C-terminus of LSP-1 in response to concanavalin A or phorbal myristate acetate stimulation of murine T cells. LSP-1 contains two N-terminal calcium-binding sites and has also been shown to bind to actin. These results suggest that LSP-1 may be involved in the binding of actin to the plasma membrane following stimulation of PKC in murine T cells. The mRNA is constitutively expressed; however, the protein is only expressed following activation of the cell. Furthermore, LSP-1 is a substrate for MAPKAP kinase 2 (MAP kinase-activated protein kinase 2) in neutrophils.