Reorganization of the cortical cytoskeleton is a hallmark of T lymphocyte activation. Upon binding to antigen presenting cells, the T cells rapidly undergo cytoskeletal re-organization thus forming a cap at the cellcell contact site leading to receptor clustering, protein segregation, and cellular polarization. Previously, we reported cloning of the human lymphocyte homologue of the Drosophila Discs Large tumor suppressor protein (hDlg). Here we show that a novel protein termed GAKIN binds to the guanylate kinase-like domain of hDlg. Affinity protein purification, peptide sequencing, and cloning of GAKIN cDNA from Jurkat J77 lymphocytes identified GAKIN as a novel member of the kinesin superfamily of motor proteins. GAKIN mRNA is ubiquitously expressed, and the predicted amino acid sequence shares significant sequence similarity with the Drosophila kinesin-73 motor protein. GAKIN sequence contains a motor domain at the NH 2 terminus, a central stalk domain, and a putative microtubule-interacting sequence called the CAP-Gly domain at the COOH terminus. Among the MAGUK superfamily of proteins examined, GAKIN binds to the guanylate kinase-like domain of PSD-95 but not of p55. The hDlg and GAKIN are localized mainly in the cytoplasm of resting T lymphocytes, however, upon CD2 receptor cross-linking the hDlg can translocate to the lymphocyte cap. We propose that the GAKIN-hDlg interaction lays the foundation for a general paradigm of coupling MAGUKs to the microtubule-based cytoskeleton, and that this interaction may be functionally important for the intracellular trafficking of MAGUKs and associated protein complexes in vivo.

Phosphatidylinositol-(3,4,5)-trisphosphate (PIP3), a product of phosphatidylinositol 3-kinase, is an important second messenger implicated in signal transduction and membrane transport. In hippocampal neurons, the accumulation of PIP3 at the tip of neurite initiates the axon specification and neuronal polarity formation. We show that guanylate kinase–associated kinesin (GAKIN), a kinesin-like motor protein, directly interacts with a PIP3-interacting protein, PIP3BP, and mediates the transport of PIP3-containing vesicles. Recombinant GAKIN and PIP3BP form a complex on synthetic liposomes containing PIP3 and support the motility of the liposomes along microtubules in vitro. In PC12 cells and cultured hippocampal neurons, transport activity of GAKIN contributes to the accumulation of PIP3 at the tip of neurites. In hippocampal neurons, altered accumulation of PIP3 by overexpression of GAKIN constructs led to the loss of the axonally differentiated neurites. Together, these results suggest that, in neurons, the GAKIN–PIP3BP complex transports PIP3 to the neurite ends and regulates neuronal polarity formation.

Human homologue of the Drosophila discs large tumor suppressor protein (hDlg) belongs to a newly discovered family of proteins termed MAGUKs that appear to have structural as well as signaling functions. Consistent with the multi-domain organization of MAGUKs, hDlg consists of three copies of the PDZ (P ᠪ SD-95/D ᠪ iscs large/z ᠪ O-1) domain, an SH3 motif, and a guanylate kinase-like domain. In addition, the hDlg contains an amino-terminal proline-rich domain that is absent in other MAGUKs. To explore the role of hDlg in cell signaling pathways, we used human T lymphocytes as a model system to investigate interaction of hDlg with known tyrosine kinases. In human T lymphocyte cell lines, binding properties of hDlg were studied by immunoprecipitation, immunoblotting, and immune complex kinase assays. Our results show that protein tyrosine kinase activity is associated with the immunoprecipitates of hDlg. Immunoblotting experiments revealed that the immunoprecipitates of hDlg contain p56 lck , a member of the Src family of tyrosine kinases. The specificity of the interaction is demonstrated by the lack of p59 fyn tyrosine kinase and phosphotidylinositol 3-kinase in the hDlg immunoprecipitates. Direct interaction between hDlg and p56 lck is demonstrated using glutathione Stransferase fusion proteins of hDlg and recombinant p56 lck expressed in the baculovirus-infected Sf9 cells. The p56 lck binding site was localized within the aminoterminal segment of hDlg containing proline-rich domain. In addition, we show in vivo association of hDlg with Kv1.3 channel, which was expressed in T lymphocytes as an epitope-tagged protein using a vaccinia virus expression system. Taken together, these results provide the first evidence of a direct interaction between hDlg and p56 lck tyrosine kinase and suggest a novel function of hDlg in coupling tyrosine kinase and voltage-gated potassium channel in T lymphocytes.