SummaryWe have previously described the isolation of a cDNA clone, designated NKG2, that was expressed in all natural killer (NK) cells tested but not in T or B cells . In the present communication, the original isolate, when used to probe a cDNA library prepared from a CD3 -NK cell clone, was found to crosshybridize with a family of transcripts that fell into four distinct groups designated NKG2-A, -B, -C, and -D. Full-length cDNA sequences were determined for each group, and the DNA and inferred peptide sequences were analyzed. All four transcripts encode type II membrane proteins of 215-233 amino acids. NKG2-A and -B peptides appear to be alternative splicing products of a single gene. NKG2-C is highly homologous with group A, having 94% homology in the external (COOH-terminal) domain and 56% homology throughout the internal and transmembrane regions . NKG2-D is distantly but significantly related (21% amino acid homology) to the first three groups . Therefore, NKG2-A, -C, and -D appear to be encoded by distinct genes within a family of NK cell-specific genes . Peptide sequence homology searches demonstrate that the NKG2 peptides are members of a supergene family that includes several other type II membrane proteins . This family is characterized by the presence of a C-type animal lectin domain, and several of its members have demonstrated transmembrane signaling capability. N K cells appear to be closely related to T cells based upon the common expression of a number of cell type-specific transcripts and cell surface antigens (1, 2). NK cells and T cells also share expression of perforin and serine proteases, components of the cytolytic machinery that are found almost exclusively in lytic cells . However, the TCR/CD3 complex responsible for triggering lytic activity in T cells is absent from NK cells, and a corresponding structure that recognizes the NK target antigen and triggers NK lytic activity has not been identified . Elucidation of these structures would greatly facilitate an understanding of the in vivo function of NK cells.We initiated a project to identify, through the use of differential hybridization and cDNA subtraction methodology, genes that are expressed preferentially in NK cells, with particular emphasis on those encoding membrane-bound proteins (1). In an earlier report, we identified 12 groups of crosshybridizing cDNA clones that were expressed in NK cells but not in an EBV transformed B cell line. One of these groups, NK group 2 (NKG2), was expressed only in NK cells among the panel of cells that we examined . In the current paper, we report that NKG2 is comprised of a group of related transcripts that encode a series of type 11 membrane proteins. Materials and MethodsA cDNA library was prepared from B22, a CD3-, CD16 -, CD56 + human NK clone that lyres K562 (1) . The library was probed with the previously described NKG2 isolate (1) to find additional cDNA clones. DNA sequences of NKG2-A, -B, and -C were determined on templates derived from the original X-Gem cDNA clones via the asymmetr...

We previously isolated a series of cDNA clones designated NKG2-A, B, C, and D from a human natural killer (NK) cell library. These transcripts encode a family of type II integral membrane proteins having an extracellular Ca(2+)-dependent lectin domain. The predicted peptides share structural similarities and amino acid sequence similarity with known receptor molecules. In this report, the genomic organization and mRNA expression of each of the genes were studied by using transcript-specific probes. Southern blot experiments reveal that the probes cross-hybridize with a maximum of five genes at high stringency. By probing a Southern blot prepared from a series of hamster/human hybrid somatic cell lines, we demonstrated that all of the hybridizing fragments occur on human chromosome 12. No gene rearrangement and little restriction fragment length polymorphism (RFLP) was observed with these probes. mRNA expression of the NKG2 genes occurred in NK cells and some T cells but not in other hematopoietic cell types or in other tissues tested. Each of the transcripts occurred in all three of the NK cell lines tested: however, the genes were differentially regulated in T cells. NKG2-D was expressed in nine of fourteen T-cell clones or lines in the panel, whereas NKG2-A/B was expressed in three and NKG2-C was expressed in only one. Expression of each of the transcripts was upregulated following T-cell growth factor (TCGF)-induced activation of a cloned NK cell. The limited distribution of these proteins and their sequence similarity with known receptor molecules suggest that they may function as receptors on human NK cells.

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We reported previously the isolation of a cDNA clone, designated NKG5, encoding a secreted protein that is expressed only in natural killer and T cells and is strongly upregulated upon cell activation. In this report we have isolated the NKG5 gene from a human placental genomic library and sequenced the gene and two kilobases of 5'-flanking DNA. Comparison with the cDNA sequence reveals that the NKG5 gene consists of five exons and four introns. Intron 1 contains a DNA segment that was reported to occur as an exon in 519, a closely related cDNA clone that was isolated from a T-cell library. This result indicates that NKG5 and 519 are alternative splicing products of a single gene. The 5'-flanking region of the NKG5 gene was analyzed for homology with the promoter regions of cytokines and other activation-induced genes showing lymphocyte-specific expression. Several segments displaying sequence similarity were identified. We also identified numerous sequence elements that have strong similarity to known binding sites for transcriptional regulatory proteins including T cell-specific and activation-specific regulatory factors. These findings are consistent with the cell-specific expression and the tight regulatory control that is observed for the NKG5 gene.

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