Abstract
Three distinct but closely related classes of receptors that bind the Fc portion of immunoglobulin G (Fc gamma RI, -II, and -III) have been identified in humans. Only Fc gamma RI has high affinity for ligand and has a unique third extracellular domain (EC3). We have characterized three genes for human Fc gamma RI (A, B, and C). Each gene consists of six exons, spans 9.4 kilobase pairs, and localizes to chromosome 1. Although they are remarkably similar, genes B and C are notably different from A; in-frame stop codons are present in the EC3 domain of genes B and C, and deletions occur in a splice donor sequence of gene B. Four distinct Fc gamma RI transcripts were analyzed. One transcript, from gene A, would encode a transmembrane receptor with three external domains. A second transcript, an alternatively spliced product of gene B, would encode a two-external domain transmembrane receptor. Two transcripts, from genes B and C, have stop codons in EC3 and would be predicted to generate secreted receptors.
Highlights
Three Genes for the Human High Affinity Fc Receptor for Immunoglobulin G (IgG) (FcrRI) Encode FourDistinct Transcription Products*
FcyRI is unique in that it expresses high affinity for ligand ( K O1, 0R-109M-’), possesses a third extracellular (EC3)Ig-like domain not seen in the othertwo-domain low affinity FcyR, and is expressed only on cells of the mononuclear phagocyte system and at low levels on neutrophilic polymorphonuclear leukocytes (PMN)
Southern blot analysis of human genomic DNA from 13 randomly selected individuals showed an additional hybridizing HindIII restriction fragmentthat was not present inthe phage 1clone used to characterize the FcyRIA gene. This observation suggested the existence of a second FcyRI gene lacking the more 5’ of the two internal HindIII sites observed in the FcyRIA gene [5]
Summary
Mers) corresponding to known exon or intron sequences, or the T7 and T3primers of pBluescript KS(+). RNase Protection Analysis-A 367-bp EcoRI to AflII fragment of A strategy for identifying additional genes was suggested cDNA p135 (nucleotides 483-844, Fig. 1A) was used as a probe in by the sequence comparison of the FcyRIA gene with the RNase protection analysiosf both DNA anRdNA. We subcloned a portion of the cDNA p135 (PEA, 367 bp from the EcoRI site in EC2 to theAflII site in EC3, Fig. L4) that was identical insequence with gene A downstream of the phage T7 promotor. RNase protection analysis was performed by hybridizing an RNA probe made from PEA to DNA isolated. The riboprobe was hybridized overnight at 45 "C with 30 pg of total RNA, and RNase protection analysis was performed asdescribed above. "'P-End-labeled HindIIIfragments of X-phage DNA were used as molecular weight markers Mannheim). "'P-End-labeled HindIIIfragments of X-phage DNA were used as molecular weight markers
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