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D Davidson

Publications and source records attributed to D Davidson.

At least 145 records · Page 8Linked to original sources
PubMed1994

Oncogenic activation of p59fyn tyrosine protein kinase by mutation of its carboxyl-terminal site of tyrosine phosphorylation, tyrosine 528.

As a result of alternative splicing, the Src-related tyrosine protein kinase p59fyn consists of two distinct isoforms termed FynB and FynT. Whereas the first product accumulates principally in brain, the second is expressed in hemopoietic cells, especially in T-lymphocytes. There is increasing evidence that the Fyn proteins are critical for normal functions of neuronal and lymphoid cells. To better understand the regulation of the catalytic function of p59fyn, we have tested the effects of mutating the major site of in vivo tyrosine phosphorylation, tyrosine 528, on the biological and biochemical properties of this enzyme. Our studies showed that a tyrosine 528-->phenylalanine (Y528F) mutation converted either Fyn isoform into a dominant oncoprotein, capable of full transformation of rodent fibroblasts. However, while both Y528F p59fynT and Y528F p59fynB were able to transform NIH 3T3 cells, activated FynT molecules were consistently more efficient at this process. It was also found that expression of wild-type p59fyn or kinase-defective Y528F Fyn molecules failed to provoke transformation of NIH 3T3 cells, implying that the transforming capabilities of Y528F Fyn relied on deregulated catalytic activity. Contrary to an earlier study (Cheng, S. H., Espino, P. C., Marshall, J., Harvey, R., Merrill, J., and Smith, A. E. (1991) J. Virol. 65, 170-179), these findings showed that mutation of the conserved carboxyl-terminal tyrosine residue markedly stimulated the catalytic function of p59fyn in vivo, implying that dephosphorylation of tyrosine 528 is sufficient to produce biologically relevant activation of the Fyn kinase. Moreover, our results provided further indication that the two Fyn isoforms possess distinct biochemical activities that may dictate functional differences in normal cell physiology.

3T3 Cells↗
PubMed1994

Unique catalytic properties dictate the enhanced function of p59fynT, the hemopoietic cell-specific isoform of the Fyn tyrosine protein kinase, in T cells.

As a result of alternative splicing, the fyn gene encodes two different tyrosine protein kinase isoforms. While one protein (p59fynB) is abundantly expressed in the brain, the alternative product (p59fynT) is contained only in cells of hemopoietic lineages, especially T lymphocytes. Sequence analyses have revealed that these two isoforms differ exclusively within a stretch of 52 amino acids which overlaps the end of the Src homology 2 (SH2) motif and the beginning of the catalytic domain. Consistent with the idea that FynT provides a specialized function in hemopoietic cells, we have previously shown that expression of activated FynT molecules, but not that of activated FynB polypeptides, enhanced the antigen responsiveness of a mouse T-cell line (BI-141) (D. Davidson, L. M. L. Chow, M. Fournel, and A. Veillette, J. Exp. Med. 175:1483-1492, 1992). In this study, we examined the basis for the distinct signalling capabilities of the two Fyn isoforms in T lymphocytes. Our biochemical analyses revealed that FynT is more adept than FynB at promoting antigen receptor-triggered calcium fluxes. This phenomenon likely contributes to the improved biological function of FynT during antigen stimulation, as the calcium ionophore ionomycin partially rescued the inability of FynB to enhance antigen-induced lymphokine secretion. To establish the structural basis for these observations, we also created and analyzed a series of chimeras of FynT and FynB. These studies demonstrated that the distinct catalytic domain of FynT, and not its altered SH2 motif, is responsible for the improved ability to augment antigen responsiveness. Similarly, this sequence enhances the ability to mobilize cytosolic calcium in response to antigen receptor stimulation. Taken together, these data show that the distinct biological impacts of FynT and FynB in T cells are related to limited structural differences in the amino-terminal portion of their catalytic domains and that they reflect, at least in part, the greater ability of FynT to mobilize cytoplasmic calcium.

Alternative Splicing↗
PubMed1994

Two distinct protein isoforms are encoded by ntk, a csk-related tyrosine protein kinase gene.

Recently, we and others have cloned cDNAs encoding a second member of the Csk family of inhibitory tyrosine protein kinases, which we have termed Ntk. Intriguingly, the mouse ntk cDNA sequences published by two independent groups differed by the presence or absence of a 136 nucleotide-insert near their 5' ends. In this report, we demonstrate that this 136 nucleotide-sequence likely corresponds to a complete exon in the ntk gene (termed exon 2), and that the two types of cDNAs/transcripts are produced by alternative splicing. Using ribonuclease protection assays, it was also established that brain and lymphoid organs, as well as most hemopoietic cells, predominantly expressed ntk transcripts lacking exon 2. In contrast, selected hemopoietic cell lines, such as the immature myeloid cell lines 32D cl3(G) and WEHI-3B, exclusively possessed exon 2-bearing RNAs. Interestingly, exon 2 introduced a novel in-frame upstream AUG in the ntk transcript, which is in the appropriate context for translation initiation. Evidence was obtained that this AUG is utilized in vivo, and that it extends the amino-terminal sequence of Ntk by 40 amino acids. Indeed, while exon 2-deficient ntk RNAs were translated into a 52 kilodalton (kDa) polypeptide (p52ntk), those bearing exon 2 produced a 56 kDa protein (p56ntk). Furthermore, p56ntk, but not p52ntk, was recognized by an antiserum directed against the novel amino-terminal sequence encoded by exon 2. Additional biochemical characterizations showed that p52ntk and p56ntk were localized to the cytoplasm, and that they partially accumulated in the detergent-insoluble cellular fraction. This last finding suggested that the Ntk proteins can associate with the cytoskeleton. Finally, through linkage analysis of two multilocus crosses, the ntk gene was mapped to Chromosome 10 in the mouse. Taken together, these data showed that ntk, a csk-related tyrosine protein kinase gene, encodes two protein isoforms expressed in distinct cell types. Moreover, they raised the possibility that Ntk may be involved in the regulation of Src-like enzymes in detergent-insoluble cellular compartments.

3T3 Cells↗
PubMed1993

Negative regulation of T-cell receptor signalling by tyrosine protein kinase p50csk.

Tyrosine protein phosphorylation is necessary for antigen receptor-mediated activation of T lymphocytes. This signal is generated at least in part by the Src-related tyrosine protein kinases p56lck and p59fynT (refs 2, 3). The activity of these two enzymes is repressed by phosphorylation of a conserved carboxy-terminal tyrosine residue. Recent studies suggest that this inhibitory phosphorylation may be caused by p50csk (for C-terminal Src kinase), a tyrosine protein kinase which accumulates most abundantly in thymus and spleen. To investigate the function of Csk in T lymphocytes and characterize the processes regulating T-cell receptor (TCR) signalling, we examined the effects of overexpression of Csk on the physiology of an antigen-specific mouse T-cell line. We report here that p50csk negatively regulates TCR-induced tyrosine protein phosphorylation and lymphokine production. This provides evidence for the involvement of Csk in the regulation of T-cell activation.

Animals↗
PubMed1993

Shared sacrifice: the right message for America.

Is President Clinton naive to ask Americans to buy into "shared sacrifice"? Actually, shared sacrifice seems the right message for America in the 1990s, writes Dick Davidson, president of the AHA. In fact, he says, it's probably the only way we'll be able to repair our economy and our health care system.

Community-Institutional Relations↗