A new member of the immunoglobulin superfamily that has a cytoplasmic region homologous to the leukocyte common antigen

Cloning and chromosomal assignment of a widely expressed human receptor-like protein-tyrosine phosphatase

Insight into the regulation of the actions of the protein-tyrosine kinases will be greatly facilitated by the full characterization of the family of protein-tyrosine phosphatases. A search for novel phosphatases resulted in the isolation of a cDNA, termed HLPR, encoding a member of the family of human receptor-like protein-tyrosine phosphatases: its cDNA sequence predicts a protein of 793 amino acids (unglycosylated Mr 87,500) and includes a 121 residue extracellular domain, a single transmembrane segment, and and two tandem intra-cytoplasmic catalytic domains. The HLPR genes is located on human chromosome 20, and the protein it encodes likely plays a fundamental role in the physiology of all cells as its expression appears to be ubiquitous.

Novel putative protein tyrosine phosphatases identified by the polymerase chain reaction

Protein tyrosine phosphatases (PTPases) are a family of enzymes that specifically dephosphorylate phosphotyrosyl residues in selected protein substrates. To more fully understand the regulatory role of protein tyrosine phosphorylation and dephosphorylation in cellular signal transduction, characterization of PTPases is essential. Using the polymerase chain reaction and degenerate oligonucleotide primers corresponding to conserved amino acid sequences within the catalytic domain of PTPases, we have identified 11 PTPase-related human liver cDNA sequences. Five of these have not been described previously. These results indicate that, like protein tyrosine kinases, PTPases may also comprise a gene family with a large number of members.

Structural diversity and evolution of human receptor-like protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPases), together with protein tyrosine kinases, regulate the tyrosine phosphorylation that controls cell activities and proliferation. Previously, it has been recognized that both cytosolic PTPases and membrane associated, receptor-like PTPases exist. In order to examine the structural diversity of receptor-like PTPases, we isolated human cDNA clones that cross-hybridized to a Drosophila PTPase cDNA clone, DPTP12, under non-stringent hybridization conditions. The cDNA clones thus isolated included LCA and six other novel receptor-like PTPases, named HPTP alpha, beta, gamma, delta, epsilon, and zeta. The cytoplasmic regions of HPTP alpha and epsilon are highly homologous, and are composed of two tandemly duplicated PTPase-like domains. The extracellular regions of HPTP alpha and epsilon are, respectively, 123 amino acids and 27 amino acids, and do not have obvious similarity to any known protein. The cytoplasmic region of HPTP beta contains only one PTPase domain. The extracellular region of HPTP beta, which is 1599 amino acids, is composed of 16 fibronectin type-III repeats. HPTP delta is very similar to leukocyte common antigen related molecule (LAR), in both the extracellular and cytoplasmic regions. Partial sequences of HPTP gamma and zeta indicate that they are highly homologous and contain two PTPase-like domains. The PTPase-like domains of HPTP alpha, beta and delta expressed in Escherichia coli had tyrosine phosphatase activities.

Cloning of three human tyrosine phosphatases reveals a multigene family of receptor-linked protein-tyrosine-phosphatases expressed in brain

A human brainstem cDNA library in bacteriophage lambda gt11 was screened under conditions of reduced hybridization stringency with a leukocyte common antigen (LCA) probe that spanned both conserved cytoplasmic domains. cDNA encoding a receptor-linked protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48), RPTPase alpha, has been cloned and sequenced. Human RPTPase alpha consists of 802 amino acids. The extracellular domain of 150 residues includes a hydrophobic signal peptide and eight potential N-glycosylation sites. This is followed by a transmembrane region and two tandemly repeated conserved domains characteristic of all RPTPases identified thus far. The gene for RPTPase alpha has been localized to human chromosome region 20pter-20q12 by analysis of its segregation pattern in rodent-human somatic cell hybrids. Northern blot analysis revealed the presence of two major transcripts of 4.3 and 6.3 kilobases. In addition to RPTPase alpha, two other RPTPases (beta and gamma), identified in the same screen, have been partially cloned and sequenced. Analysis of sequence comparisons among LCA, the LCA-related protein LAR, and RPTPases alpha, beta, and gamma reveals the existence of a multigene family encoding different RPTPases, each containing a distinct extracellular domain, a single hydrophobic transmembrane region, and two tandemly repeated conserved cytoplasmic domains.

Expression of a human T-cell protein-tyrosine-phosphatase in baby hamster kidney cells

A human T-cell cDNA encoding a 48-kDa protein-tyrosine-phosphatase (PTPase; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) was cloned into a mammalian expression vector and introduced into baby hamster kidney cells, and stable colonies were isolated. The expressed PTPase was found to be associated with the particulate fraction of the cells, where it was essentially inactive in an in vitro assay unless first subjected to limited trypsinization; trypsin treatment generated an active fragment of 33 kDa by the removal of a carboxyl-terminal segment of the full-length enzyme. Gel filtration indicated that the expressed enzyme was associated with a complex of greater than 600 kDa. Introduction of a premature stop codon into the T-cell cDNA at position 1012 resulted in the production of a fully active 37-kDa species that distributed between both the particulate and soluble fractions. The truncated form of the enzyme was readily solubilized by detergents and was eluted within its predicted molecular mass range. These results suggest that the carboxyl-terminal segment is important in determining the localization and regulation of the PTPase. The level of protein-tyrosine phosphorylation observed after 5 min of platelet-derived growth factor stimulation was reduced in cells overexpressing either form of the phosphatase, indicating that both are active in vivo. Overexpressing the truncated enzyme resulted in a growth rate that was approximately 50% of that observed in cells transfected with either the full-length PTPase cDNA or the vector alone.

Protein tyrosine phosphatase activity of an essential virulence determinant in Yersinia

Yersinia is the genus of bacteria that is the causative agent in plague or the black death, and on several occasions this organism has killed a significant portion of the world's population. An essential virulence determinant of Yersinia was shown to be a protein tyrosine phosphatase. The recombinant 50-kilodalton Yersinia phosphatase had a specificity for removal of phosphate from Tyr-containing as opposed to Ser/Thr-containing phosphopeptides and proteins. Site-directed mutagenesis was used to show that the Yersinia phosphatase possesses an essential Cys residue required for catalysis. Amino acids surrounding an essential Cys residue are highly conserved, as are other amino acids in the Yersinia and mammalian protein tyrosine phosphatases, suggesting that they use a common catalytic mechanism.

Cloning and expression of a widely expressed receptor tyrosine phosphatase

We describe the identification of a widely expressed receptor-type (transmembrane) protein tyrosine phosphatase (PTPase; EC 3.1.3.48). Screening of a mouse brain cDNA library under low-stringency conditions with a probe encompassing the intracellular (phosphatase) domain of the CD45 lymphocyte antigen yielded cDNA clones coding for a 794-amino acid transmembrane protein [hereafter referred to as receptor protein tyrosine phosphatase alpha (R-PTP-alpha)] with an intracellular domain displaying clear homology to the catalytic domains of CD45 and LAR (45% and 53%, respectively). The 142-amino acid extracellular domain (including signal peptide) of R-PTP-alpha is marked by a high serine/threonine content (32%) as well as eight potential N-glycosylation sites but displays no similarity to known proteins. Genetic mapping assigns the gene for R-PTP-alpha to mouse chromosome 2, closely linked to the Il-1a and Bmp-2a loci. The corresponding mRNA (3.0 kilobases) is expressed in most murine tissues and most abundantly expressed in brain and kidney. Antibodies against a synthetic peptide of R-PTP-alpha identified a 130-kDa protein in cells transfected with the R-PTP-alpha cDNA.

The axonal glycoprotein TAG-1 is an immunoglobulin superfamily member with neurite outgrowth-promoting activity

Pathfinding of axons in the developing nervous system is thought to be mediated by glycoproteins expressed on the surface of embryonic axons and growth cones. One molecule suggested to play a role in axonal growth is TAG-1, a 135 kd glycoprotein expressed transiently on the surface of subsets of neurons in the developing mammalian nervous system. We isolated a full-length cDNA clone encoding rat TAG-1. TAG-1 has six immunoglobulin-like domains and four fibronectin type III-like repeats and is structurally similar to other immunoglobulin-like proteins expressed on developing axons. Neurons maintained in vitro on a substrate of TAG-1 extend long neurites, suggesting that this protein plays a role in the initial growth and guidance of axons in vivo. TAG-1 is anchored to the neuronal membrane via a glycosyl phosphatidylinositol linkage and is also released from neurons, suggesting that TAG-1 also functions as a substrate adhesion molecule when released into the extracellular environment.

Cloning of a cDNA for a major human protein-tyrosine-phosphatase

We have isolated a cDNA clone encoding the major protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) of human placenta. Degenerate oligonucleotides, based on the amino acid sequence of the protein, were used to amplify an internal fragment of the gene from human placental cDNA by the polymerase chain reaction. This fragment was then used to probe a human placental cDNA library. A 3.3-kilobase (kb) insert was isolated and sequenced. The insert has a single extended open reading frame that predicts a 435 amino acid protein of Mr approximately 50,000. From the amino terminus to residue 321, the deduced amino acid sequence is identical to that previously determined by peptide sequencing [Charbonneau, H., Tonks, N. K., Kumar, S., Diltz, C. D., Harrylock, M., Cool, D. E., Krebs, E. G., Fischer, E. H. & Walsh, K. A. (1989) Proc. Natl. Acad. Sci. USA 86, 5252-5256]; however, the sequence predicts that the protein contains an additional 114 amino acids not present in the reported peptide sequence. In vitro translation of the 3.3-kb insert produces a protein of Mr 56,000, in general agreement with the predicted size. The phosphatase gene appears to be present as a single copy in human genomic DNA and is transcribed into a 3.5-kb message in a variety of tissues.

Identification of a chromosome 18q gene that is altered in colorectal cancers

Allelic deletions involving chromosome 18q occur in more than 70 percent of colorectal cancers. Such deletions are thought to signal the existence of a tumor suppressor gene in the affected region, but until now a candidate suppressor gene on this chromosomal arm had not been identified. A contiguous stretch of DNA comprising 370 kilobase pairs (kb) has now been cloned from a region of chromosome 18q suspected to reside near this gene. Potential exons in the 370-kb region were defined by human-rodent sequence identities, and the expression of potential exons was assessed by an "exon-connection" strategy based on the polymerase chain reaction. Expressed exons were used as probes for cDNA screening to obtain clones that encoded a portion of a gene termed DCC; this cDNA was encoded by at least eight exons within the 370-kb genomic region. The predicted amino acid sequence of the cDNA specified a protein with sequence similarity to neural cell adhesion molecules and other related cell surface glycoproteins. While the DCC gene was expressed in most normal tissues, including colonic mucosa, its expression was greatly reduced or absent in most colorectal carcinomas tested. Somatic mutations within the DCC gene observed in colorectal cancers included a homozygous deletion of the 5' end of the gene, a point mutation within one of the introns, and ten examples of DNA insertions within a 0.17-kb fragment immediately downstream of one of the exons. The DCC gene may play a role in the pathogenesis of human colorectal neoplasia, perhaps through alteration of the normal cell-cell interactions controlling growth.

Integrity of the exon 6 sequence is essential for tissue-specific alternative splicing of human leukocyte common antigen pre-mRNA

By alternative splicing, exons 4, 5, and 6 of the human leukocyte common antigen (LCA) gene are included in B-cell mRNA but excluded from thymocyte mRNA. A mini-LCA gene that contains only LCA exons 2, 6, and 8 faithfully reproduces this tissue-specific alternative splicing in mouse B and thymocyte cell lines. Elimination of almost all of the intron sequences associated with exon 6 had no effect on the alternative splicing, while linker-scanning analysis showed that a significant length of the exon 6 sequence is essential for alternative splicing.

Protein tyrosine dephosphorylation and signal transduction

The protein tyrosine phosphatases comprise a family of enzymes that specifically dephosphorylate tyrosyl residues. Determination of the amino acid sequence of a major low molecular mass form isolated from human placenta (PTPase 1B) provided the basis for the first identification of transmembrane proteins that bear intracellular phosphatase domains. The existence of such molecules, bearing the hallmarks of receptors, raises the exciting possibility of a novel mechanism of signal transduction in which the early events involve the ligand-induced dephosphorylation of tyrosyl residues in proteins.

A family of receptor-linked protein tyrosine phosphatases in humans and Drosophila

To understand the regulation of cell proliferation by tyrosine phosphorylation, characterization of protein tyrosine phosphatases (PTPase; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) is essential. The human genes LCA (leukocyte common antigen) and LAR encode putative receptor-linked PTPases. By using consensus sequence probes, two additional receptor-linked PTPase genes, DLAR and DPTP, were isolated from Drosophila melanogaster. The extracellular segments of both DLAR and DPTP are composed of multiple immunoglobulin-like domains and fibronectin type III-like domains. The cytoplasmic region of DLAR and DPTP, as well as human LCA and LAR, are composed of two tandemly repeated PTPase domains. PTPase activities of immunoprecipitated LCA and LAR were demonstrated by measuring the release of phosphate from a 32P-labeled [Tyr(P)]peptide. Furthermore, the cytoplasmic domains of LCA, LAR, DLAR, and DPTP, expressed in Escherichia coli, have PTPase activity. Site-directed mutagenesis showed that a conserved cysteine residue is essential for PTPase activity.

Integrity of the exon 6 sequence is essential for tissue-specific alternative splicing of human leukocyte common antigen pre-mRNA

By alternative splicing, exons 4, 5, and 6 of the human leukocyte common antigen (LCA) gene are included in B-cell mRNA but excluded from thymocyte mRNA. A mini-LCA gene that contains only LCA exons 2, 6, and 8 faithfully reproduces this tissue-specific alternative splicing in mouse B and thymocyte cell lines. Elimination of almost all of the intron sequences associated with exon 6 had no effect on the alternative splicing, while linker-scanning analysis showed that a significant length of the exon 6 sequence is essential for alternative splicing.

Evidence that the leukocyte-common antigen is required for antigen-induced T lymphocyte proliferation

The leukocyte-common antigen (L-CA) is a family of large molecular weight glycoproteins uniquely expressed on the surface of all nucleated cells of hematopoietic origin. The glycoprotein consists of a heavily glycosylated exterior domain, a single membrane spanning region, and a large cytoplasmic domain that contains tyrosine phosphatase activity. To investigate the function of this family, we generated T cell clones that lacked L-CA (L-CA-). The expression of the alpha beta T cell receptor, CD3, CD4, IL-2 receptor (p55), LFA-1, Thy-1, and Pgp-1 (CD44) was normal. The L-CA- T cell clones failed to proliferate in response to antigen or cross-linked CD3; however, they could still proliferate in response to IL-2. An L-CA+ revertant was obtained and the ability to proliferate in response to antigen and cross-linked CD3 was restored. These data indicate that L-CA is required for T cells to enter into cell cycle in response to antigen.

Human placenta protein-tyrosine-phosphatase: amino acid sequence and relationship to a family of receptor-like proteins

The amino acid sequence of the cytosolic human placenta protein-tyrosine-phosphatase 1B (PTPase 1B; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) has been determined. It consists of a single chain of 321 residues with an N-acetylated N-terminal methionine and an unusually proline-rich C-terminal region. The enzyme is structurally related to the two cytoplasmic domains of both the leukocyte common antigen CD45 and LAR, a CD45-like molecule with an external segment that resembles a neural cell adhesion molecule. A low molecular weight protein encoded by a cDNA clone from T cells also shows extensive sequence similarities. The present study defines homologous domains common to this diverse family of PTPases that includes both soluble and receptor-like transmembrane forms. The cysteinyl residues 121 and 215 of PTPase 1B are conserved among all members of the family and are candidates for involvement in catalysis since PTPase 1B is inactivated by thiol modifying reagents. Two segments rich in positively charged residues (residues 33-47 and 227-238) may provide sites of interaction with inhibitory anionic polymers such as heparin or poly(Glu/Tyr).

cDNA isolated from a human T-cell library encodes a member of the protein-tyrosine-phosphatase family

A human peripheral T-cell cDNA library was screened with two labeled synthetic oligonucleotides encoding regions of a human placenta protein-tyrosine-phosphatase (protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48). One positive clone was isolated and the nucleotide sequence was determined. It contained 1305 base pairs of open reading frame followed by a TAA stop codon and 978 base pairs of 3' untranslated end, although a poly(A)+ tail was not found. An initiator methionine residue was predicted at position 61, which would result in a protein of 415 amino acid residues (Mr, 48,400). This was supported by the synthesis of a Mr 48,000 protein in an in vitro reticulocyte lysate translation system using RNA transcribed from the cloned cDNA and T7 RNA polymerase. The deduced amino acid sequence was compared to other known proteins revealing 65% identity to the low Mr PTPase 1B isolated from placenta. In view of the high degree of similarity, the T-cell cDNA likely encodes a newly discovered protein-tyrosine-phosphatase, thus expanding this family of genes.

Leukocyte cell surface enzymology: CD45 (LCA, T200) is a protein tyrosine phosphatase

During 1987, striking advances were made in defining the receptors and ligands for cell-to-cell adhesion interactions involving leukocytes. In 1988, two major leukocyte differentiation antigens, CD10 (cALLA) and CD45 (LCA, T200), were shown to be enzymes while two other markers, CD4 and CD8, were found to be associated with an enzyme. In this article, Ed Clark and Jeff Ledbetter discuss recent findings in the emerging area of leukocyte cell surface enzymology with emphasis on CD45, a membrane-associated protein tyrosine phosphatase (PTPase)2,3.

Kinases and phosphatases in T-cell activation

Protein phosphorylation-dephosphorylation plays an important role in signal transduction in T lymphocytes. In this review, Denis Alexander and Doreen Cantrell focus on the identification, regulation and functions of the kinases and phosphatases that control phosphorylation events in T cells.