The protein tyrosine kinase p56lck inhibits CD4 endocytosis by preventing entry of CD4 into coated pits.

The lymphocyte glycoprotein CD4 is constitutively internalized and recycled in nonlymphoid cells, but is excluded from the endocytic pathway in lymphocytic cells (Pelchen-Matthews, A., J. E. Armes, G. Griffiths, and M. Marsh. 1991. J. Exp. Med. 173: 575-587). Inhibition of CD4 endocytosis is dependent on CD4 expressing an intact cytoplasmic domain and is only observed in cells where CD4 can interact with the protein tyrosine kinase p56lck, a member of the src gene family. We have expressed p56lck, p60c-src, or chimeras of the two proteins in CD4-transfected NIH-3T3 or HeLa cells. Immunoprecipitation of CD4 and in vitro kinase assays showed that p56lck and the lck/src chimera, which contains the NH2 terminus of p56lck, can associate with CD4. In contrast, p60c-src and the src/lck chimera, which has the NH2 terminus of p60c-src, do not associate with CD4. Endocytosis assays using radioiodinated anti-CD4 monoclonal antibodies demonstrated that coexpression of CD4 with p56lck, but not with p60c-src, inhibited CD4 endocytosis, and that the extent of the inhibition depended directly on the relative levels of CD4 and p56lck expressed. The uptake of mutant CD4 molecules which cannot interact with p56lck was not affected. Measurement of the fluid-phase endocytosis of HRP or the internalization of transferrin indicated that the effect of p56lck was specific for CD4, and did not extend to other receptor-mediated or fluid-phase endocytic processes. Immunogold labeling of CD4 at the cell surface and observation by electron microscopy demonstrated directly that p56lck inhibits CD4 endocytosis by preventing its entry into coated pits.

Two isoforms of murine hck, generated by utilization of alternative translational initiation codons, exhibit different patterns of subcellular localization.

Mammalian hck, a member of the src family of tyrosine kinases, is expressed predominantly in cells of the myeloid and B-lymphoid lineages. Using mutational analysis, we have investigated the molecular basis of two immunoreactive forms of murine hck of 56 and 59 kDa found in numerous hemopoietic cell types. Our results indicate that translation of murine p59hck initiates from a CTG codon located 21 codons 5' of an ATG that is utilized to generate p56hck. We provide evidence that two human hck isoforms are generated by the same mechanism. Subcellular fractionation studies reveal that while p59hck and p56hck are associated with membranes of various murine B-lymphoid and myeloid cell lines, p59hck alone is also located in the cytosol. In contrast to membrane-associated p59hck, which is metabolically labeled with [3H]myristic acid and exhibits amphiphilic properties in Triton X-114 detergent, cytosolic p59hck is hydrophilic, suggesting that it is not acylated. Possible mechanisms are proposed to account for these observations.

Alternatively spliced murine lyn mRNAs encode distinct proteins.

Two lyn proteins of 56 and 53 kDa have been observed in immunoprecipitates from a variety of murine and human cell lines and tissues. We report the cloning and nucleotide sequence of two distinct murine lyn cDNAs isolated from an FDC-P1 cDNA library. One of the cDNAs, designated lyn11, encodes a protein of 56 kDa which shares 96% similarity with human lyn. The other cDNA, designated lyn12, encodes a protein of 53 kDa. The proteins differ in the presence or absence of a 21-amino-acid sequence located 24 amino acids C terminal of the translational initiation codon. Using RNase protection analysis, we have identified mRNAs corresponding to both cDNAs in murine cell lines and tissues. Sequence analysis of murine genomic clones suggests that the distinct mRNAs are alternatively spliced transcripts derived from a single gene. Expression of both cDNAs in COS cells leads to the production of lyn proteins with the same molecular weight as the two forms of lyn proteins immunoprecipitated from extracts of FDC-P1 cells and mouse spleen. Subcellular fractionation studies and Western immunoblotting analysis suggest that both isoforms of lyn are membrane associated. The association of both lyn isoforms with the membrane fraction supports the notion that lyn, like other src-related kinases, may interact with the intracellular domain of cell surface receptors.

Elevated level of p60c-src in virus-transformed murine megakaryocytic cell lines.

Megakaryocytic cell lines derived from mouse bone marrow cells transformed by the Myeloproliferative Leukemia Virus (MPLV) contain elevated levels of p60c-src. Northern blot analysis revealed the presence of a 4 kb normal sized c-src transcript only in MPLV-transformed megakaryocytic cell lines containing a high percentage of acetylcholinesterase positive cells (AChE+ greater than 10%), but not in MPLV-transformed erythroblastic or myeloblastic cell lines. The p60c-src protein was identified in lysates from in vivo labelled cells and in in vitro labelled membrane extracts by immunoblotting analysis and by immunoprecipitations with specific anti-src antibodies. In dimethylsulfoxide (DMSO) treated cells, the number of AChE+ cells increased together with p60c-src kinase activity indicating a possible correlation between p60c-src expression/activity and megakaryocytic differentiation.

Lipopolysaccharide- and interferon-gamma-induced expression of hck and lyn tyrosine kinases in murine bone marrow-derived macrophages.

We have examined the role of tyrosine phosphorylation during the course of macrophage activation. Initial experiments indicated that vanadate, a known phosphotyrosine phosphatase inhibitor, enhanced the phorbol 12-myristate 13-acetate (PMA)-triggered respiratory burst and potentiated the priming effects of bacterial lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma), suggesting that tyrosine phosphorylation may be important in these end cell functions. As src-related kinases have been implicated in the activation of cells of other haemopoietic lineages, we examined the relationship between the activity of two such kinases, hck and lyn, and priming of the respiratory burst. We found that the level of hck and lyn is increased following exposure of bone marrow-derived macrophages (BMM) to LPS or IFN-gamma. The induction of both of these kinases follows similar kinetics with maximal activity occurring at 24-48 h. Interestingly, the kinetics of induction of hck and lyn kinase activity in BMM demonstrated a close temporal relationship with the priming effects of LPS and IFN-gamma on the macrophage respiratory burst. Collectively, these observations raise the possibility that modulation of expression of hck and lyn is involved in the regulation of the respiratory burst.

The retarded electrophoretic migration of p56lck induced by vanadate in lymphoma cells correlates with modified kinase activity.

p56lck is a src related lymphocyte specific tyrosine protein kinase which undergoes specific changes during T-cell activation, particularly the appearance of slow migrating forms. To analyze these forms, LSTRA cells were treated with vanadate. This resulted in increased phosphorylation of p56lck with the appearance of slow migrating forms. Renaturation of the p56lck bands after gel migration showed that vanadate mostly increased the activity of the lower band of p56lck. The upper bands had a reduced specific activity. In addition, the upper bands from vanadate treated cells displayed additional phosphorylated sites.

Correlation between phosphorylation and kinase activity of a tyrosine protein kinase: p56lck.

P56lck is the product of a cellular oncogene highly expressed in lymphoid cells. Tyrosine kinase activity was measured by using an exogenous substrate: polyamino acid glutamic acid-tyrosine (4:1) (PGT). Different levels of phosphorylation of p56lck were achieved by the utilisation of SH reagents and different lengths of incubation time. The phosphorylation of PGT was proportional to the level of phosphorylation of p56lck. Identical results were obtained with crude membrane preparations and with p56lck partially purified on immunecomplexes.

The amino terminal region of the p56 lck from LSTRA exerts negative modulation on the tyrosine kinase activity.

High levels of tyrosine kinase activity have been detected in the murine lymphoma LSTRA (p56). The functional domains of this kinase have been studied by the use of antibodies generated against peptides from the amino terminal region and from the tyrosine autophosphorylation site. The amino terminal antibody had higher affinity for the p56 than the antibody directed against the phosphotyrosine site. However, the phosphorylation of exogenous substrate by p56 was lower when the tyrosine kinase was immunocomplexed by the antibody against the amino terminal region than when the kinase was complexed by the phosphorylation site antibody. This suggests that in the N-terminal region exist structures which modulate the tyrosine kinase activity of the p56.

Phosphorylation of p56lck by external ATP in intact cells.

Recent studies have suggested a role for extracellular ATP. In this report we show that extracellular labelled ATP crosses the plasma membrane of intact lymphoma cells and peripheral blood lymphocytes and phosphorylates p56lck a tyrosine protein kinase specific of lymphoid cells. Two other phosphoproteins of 92Kd and 35Kd become detectable on alkali treated gels. Phosphorylation occurs within minutes following addition of ATP. ATP, GTP, ADP and an ATP analog prevent phosphorylation but not AMP nor Pi; trypsinization of cells abolishes labelling. The possible involvement of P2 purinergic receptors is discussed.

Purification and characterization of a higher-molecular-mass form of protein phosphotyrosine phosphatase (PTP 1B) from placental membranes.

Purification of a major placental membrane protein phosphotyrosine phosphatase (PTP-I) through the use of a nonhydrolysable phosphotyrosine analogue affinity ligand has enabled identification of the enzyme as a single polypeptide of at least 46 kDa. This phosphatase specifically dephosphorylates phosphotyrosine-containing substrates, including the src peptide, the epidermal-growth-factor receptor tyrosine kinase and the non-receptor tyrosine kinase p56lck. The p56lck can be dephosphorylated by PTP-I at two tyrosine residues (Tyr-394 and Tyr-505), which are differentially phosphorylated in vitro and in vivo and have been suggested to modulate kinase activity. The activity of PTP-I towards these substrates indicates a possible function of regulation of cellular tyrosine phosphorylation pathways at the level of growth-factor receptor and/or oncogene/proto-oncogene tyrosine kinases. Kinetic analyses show that PTP-I exhibits a Km value of about 2 microM with either src peptide or reduced, carboxyamidomethylated and maleylated (RCM)-lysozyme as substrate, and is inhibited in a mixed competitive manner by the polyanions heparin and poly(Glu4,Tyr1). Sequencing of PTP-I peptides reveals almost complete identity with sequences within the N-terminal half of the 37 kDa non-receptor tyrosine phosphatase 1B. However, the size and amino acid composition of PTP-I are similar to that of a higher-molecular-mass form of PTP 1B predicted from cDNA cloning. These results suggest that the 37 kDa PTP 1B is a proteolysed form of PTP-I, and provide evidence that a larger form of PTP 1B exists in vivo, at least in association with placental membranes.