Robert L.Geahlen
Professor; Ph.D., California, San Diego, 1977
Signals
transduced by the binding of antigens to receptors on hematopoietic cells take
the form of increased tyrosine phosphorylation of intracellular proteins. Since
the components of the receptor complex itself lack intrinsic enzymatic activity,
these responses are mediated instead by cytoplasmic protein-tyrosine kinases
that associate with the liganded receptor. The major focus of our laboratory
is on understanding how these kinases are regulated and how, once activated,
they regulate the multiple biochemical pathways that are activated by receptor
engagement. Kinases of particular interest to our laboratory include Syk, a
72 kDa protein-tyrosine kinase, and Lyn, a member of the Src-family of enzymes.
We use chemical, biochemical and genetic approaches to understanding the role
of each of these enzymes in signal transduction pathways downstream from both
B cell antigen receptors and mast cell IgE receptors. Our current work is directed
in several areas.
Structure-Function Analysis of Syk Function in Hematopoietic Cells. B cells
fail to develop properly in mice lacking the gene for Syk due to the inability
of pre-B cell antigen receptors to signal in the absence of Syk. How Syk mediates
signaling through B cell antigen receptors is a major question being investigated.
We have mapped multiple sites of phosphorylation on Syk that are important for
its ability to function in B cells. By site-directed mutagenesis and expression
studies, we are exploring the role of these phosphorylations in the ability of
Syk to complement signaling in Syk-deficient cells.
Interactions of Syk With Intracellular Proteins. Once activated,
Syk dissociates from the antigen receptor and interacts with cytosolic
proteins including tubulin, which it phosphorylates on a single
tyrosine located near the carboxyl terminus of the alpha-subunit.
We are currently investigating the consequences of this phosphorylation
on tubulin function and microtubule formation.
Inhibitors of Protein-Tyrosine Kinases. We are interested
in the development of chemical probes that act selectively to inhibit
the activity of cytoplasmic protein-tyrosine kinases. Targets include
the kinase active sites themselves, as well as acyltransferases
that catalyze the co- and post-translational acylation of Src-family
kinases. These projects involve collaborations with members of
the Department and include mechanism-based screening of natural
products and synthetic design of small molecule inhibitors.
H. Ma, T.M. Yankee, D.J. Asai, M.L. Harrison, and R.L. Geahlen, "Visualization of Syk-antigen receptor interactions using green fluorescent protein: Differential roles for Syk and Lyn in the regulation of receptor capping and internalization," J. Immunol. 166, 1507-1516 (2001).
S. Faruki, R.L. Geahlen, and D.J. Asai, "Syk-dependent phosphorylation of microtubules in activated B-lymphocytes," J. Cell Sci. 113, 2557-2565 (2000).
B.S. Gaul, M.L. Harrison, R.L. Geahlen, R.A. Burton, and C.B. Post, "Substrate recognition by the Lyn protein-tyrosine kinase: NMR structure of the immunoreceptor tyrosine-based activation motif signaling region of the B-cell antigen receptor," J. Biol. Chem. 275, 16174-16182 (2000).
T. M. Yankee, L.M. Keshvara, S. Sawasdikosol, M. L. Harrison, and R.L. Geahlen, "Inhibition of Signaling Through the B Cell Antigen Receptor by the Proto-oncogene Product, c-Cbl, Requires Syk Tyrosine 317 and the c-Cbl Phosphotyrosine-binding Domain," J. Immunol. 163, 5827-5835 (1999).
Stupack, D.G., Li, E., Silletti, S., Kehler, J.A., Geahlen, R.L., Hahn, K., Nemerow, G.R., and Cheresh, D.A. "Matrix Valency Regulates Integrin-mediated Lymphoid Adhesion Via Syk Kinase," J. Cell Biol. 144, 777-788 (1999).
Miller, L.A., Kinch, M.S., Hong, J., Harrison, M.L., and Geahlen, R.L. "The Engagement of Beta-1 integrins on promonocytic cells promotes phosphorylation of Syk and formation of a protein complex containing Lyn and Beta-1 integrin," Eur. J. Immunol. 29, 1426-1434 (1999).
Fernandez, J.A., Keshvara, L.M., Peters, J.D., Furlong, M.T. Harrison, M.L. and Geahlen, R.L. "Phosphorylation- and Activation-Independent Association of the Tyrosine Kinase Syk and the Tyrosine Kinase Substrates Cbl and Vav with Tubulin in B-Cells" J. Biol. Chem. 274, 1401-1406 (1999).