Molecular cloning of transmembrane and soluble forms of a novel rat natural killer cell receptor related to 2B4

Natural killer (NK)-cell recognition of target cells and cytolytic function are controlled by multiple receptor-ligand interactions. These receptors can transmit either positive or negative signals and belong to the lectin superfamily or immunoglobulin superfamily (IgSF). One member of the IgSF, 2B4, is expressed on the surface of all mouse and human NK cells and the subset of T cells that mediate NK-like killing. In both mouse and human, 2B4 is a transmembrane protein and is the counter-receptor for CD48. Northern blot analysis had indicated the existence of 2B4-related genes. Here we report the cloning of novel cDNAs (r2B4R) closely related to the rat 2B4. Unlike 2B4, rat NK cells express mRNA corresponding to both transmembrane (r2B4R-tm) and soluble (r2B4R-se) forms of r2B4R. r2B4R-tm contains an open reading frame encoding a polypeptide of 311 amino acid residues. The encoded protein has characteristics of type I transmembrane proteins with a 20-amino acid leader sequence, a 203-amino acid extracellular domain, a 23-amino acid transmembrane domain, and a 65-amino acid cytoplasmic domain. r2B4R-se encodes a protein of 205 amino acid residues without a putative transmembrane domain. Northern blot analysis and reverse transcriptase-PCR analysis revealed that both transmembrane and soluble forms of r2B4R are expressed in interleukin-2-activated NK cells.