Description
Fc Receptor-Like 5 (FCRL5), also known as FcRH5, IRTA2, and CD307, is a 120kD protein with sequence homology to classical Fc receptors. The type 1 transmembrane FCRL proteins contain from three to nine immunoglobulin-l i ke domains. They are differentially expressed within the B cell lineage and can either promote or inhibit B cell proliferation and activation. According to testing, FCRL5 binds to purified human IgG with high affinity. Mature human FCRL5 consists of a 836aa extracellular domain (ECD) with nine Ig-like domains, a 21aa transmembrane segment, and a 105aa cytoplasmic domain with one immunotyrosine activation motif (ITAM) and two immunotyrosine inhibitory motifs (ITIMs). Mouse FCRL5 contains only five Ig-like domains in its ECD. It shares 49% aa sequence identity with human FCRL5 within common regions. Alternate splicing of human FCRL5 generates isoforms that consist of approximately the first one, six, or eight Ig-like domains. FCRL5 expression is restricted to mature B lineage cells in lymphoid tissues and blood. Its ligation inhibits signaling through the B cell antigen receptor. Epstein-Barr virus transformation of B cells induces the upregulation of surface FCRL5 by a direct effect of its EBNA2 protein on FCRL5 gene transcription. The FCRL5 gene maps to the 1q21 chromosomal locus, a common site of rearrangements in B cell malignancies, and the FCRL5 protein is preferentially expressed in cell lines with 1q21 abnormalities. FCRL5 is upregulated on tumor cells in some types of B cell malignancies. In addition, soluble FCRL5 is elevated in the serum of many B cell leukemia patients