Definition of an Fc receptor-related gene (FcRX) expressed in human and mouse B cells

The recent identification of five human Fc receptor (FcR) homologs, hFcRH1-5, has extended the known FcR family and identified an unanticipated richness of the chromosome 1q region in genes encoding potential Ig-binding proteins. In a database search for additional relatives of this family we identified expressed sequence tag representatives of a new FcR-related molecule (hFcRX) and its mouse ortholog (mFcRX). The FcRX cDNAs were cloned from human lymph node and mouse spleen cDNA libraries. hFcRX is located centromeric of FcgammaRII and FcgammaRIII at 1q23, and its mouse ortholog resides in a syntenic region of chromosome 1. The genes encode proteins with 67% interspecies identity that lack both N-linked glycosylation sites and transmembrane regions. Two of the four FcRX domains are Ig-like, and share characteristics similar to FcgammaRI domains 2 and 3, having 28% overall extracellular identity with hFcgammaRI and 27% identity with mFcgammaRI respectively. FcRX transcripts are found primarily in secondary lymphoid tissues, where they are expressed by B lineage cells. FcRX thus may function as a secreted or intracellular protein in normal and neoplastic B cells.