G6b, a Novel Immunoglobulin Superfamily Member Encoded in the Human Major Histocompatibility Complex, Interacts with SHP-1 and SHP-2

Abstract

The G6b gene, located in the class III region of the human major histocompatibility complex, has been suggested to encode a putative receptor of the immunoglobulin superfamily. Genomic sequence information was used as a starting point to clone the corresponding cDNA. Reverse transcriptase polymerase chain reaction showed that expression of the gene is restricted to certain hematopoietic cell lines including K562, Molt 4, and Jurkat. Several splice variants were detected, varying only in their C-terminal parts. One of the potential membrane-bound isoforms contained two immunoreceptor tyrosine-based inhibitory motifs in its cytoplasmic tail. Four of the isoforms were expressed as epitope-tagged proteins in the cell lines K562 and COS-7. The two splice isoforms lacking the hydrophobic transmembrane segment were secreted from the cell. Glycosidase treatment of the four recombinant proteins provided evidence for N-and O-glycosylation. Immunofluorescence studies indicated that the spliced isoforms having a transmembrane segment were directed to the cell membrane. The G6b isoform containing two immunoreceptor tyrosinebased inhibitory motifs in its cytoplasmic tail was found to be phosphorylated on tyrosine residues after pervanadate treatment of cells and, subsequently, interacts with the SH2-containing protein-tyrosine phosphatases SHP-1 and SHP-2. Mutagenesis studies showed that phosphorylation of tyrosine 211 is critical for the interaction of G6b with SHP-1 and SHP-2.The Ig superfamily receptors constitute a large group of cell surface proteins involved in the immune system and cellular recognition (1, 2). Members of this family are characterized by an extracellular part containing at least one immunoglobulin domain, a transmembrane segment, and a cytoplasmic tail. A subset of the Ig superfamily is the inhibitory receptor characterized by the presence of one or more immunoreceptor tyrosine-based inhibitory motifs (ITIM) 1 in their cytoplasmic tail.The consensus sequence of the ITIM is generally described as (L/V/I/S/T)XYXX(L/V) (3). Following phosphorylation of the tyrosine residue within this motif, the SH2 domain containing protein-tyrosine phosphatases SHP-1 and/or SHP-2 can be recruited to the receptor, where they can dephosphorylate membrane-bound phosphoproteins, thus modulating the signaling cascade. SHP-1 is a non-transmembrane protein primarily expressed in hematopoietic cells and is considered to play a negative role in cell signaling (4). Identified substrates for SHP-1 are the linker of activated T-cells (5, 6) and the adapter protein Slp-76 (7). In contrast, SHP-2 has been considered to act primarily as a positive signal transducer (8). Possible substrates for this phosphatase are the platelet-derived growth factor beta receptor (9) and PZR (10). SHP-2 modulates the signal strength of receptor-protein-tyrosine kinases and is also involved in cytokine and antigen signaling not involving receptors with intrinsic kinase activity (11). Although the two phosphatases appear to have opposing roles, there are ...

Analysis of the Gene-Dense Major Histocompatibility Complex Class III Region and Its Comparison to Mouse

“…However, they were found to contain motifs/domains that could indicate a potential function. For example, G6F and C6orf25 contain potential Ig domains, indicating that they could encode potential cell surface receptors involved in the immune system and/or inflammation (de Vet et al 2001). Similarly, C6orf46 was found to contain a Broad Complex Tramtrack and Bric Brac (BTB) motif and four Zn fingers, features of a transcription factor, whereas others such as BAT4 contain several ANK repeats, indicating potential roles in protein-protein interactions within the cell.…”

Downregulation of Platelet Responsiveness Upon Contact With LDL by the Protein-Tyrosine Phosphatases SHP-1 and SHP-2

“…Only G6b-B has ITIM motifs that when tyrosine phosphorylated, bind SHP-1 and SHP-2. 41 Cross-linking of G6b-B prior to platelet activation leads to a significant inhibition of aggregation. 36 Further research is required to elucidate which receptor is responsible for the PECAM-1-independent activation of SHP-1 and SHP-2.…”

The novel inhibitory receptor G6B is expressed on the surface of platelets and attenuates platelet function in vitro

“…1 The gene encodes a number of splice forms that are translated into both cell-surface and secreted isoforms. 2 The 2 principle cell-surface isoforms (G6B-A and G6B-B) have the same extracellular N-terminal Ig-like domain but differ in their Cterminal cytoplasmic tails. The G6B-B isoform has previously been shown to have 2 immunoreceptor tyrosine-based inhibitory motifs (ITIMs) within its cytoplasmic tail, suggesting an inhibitory role.…”