Trk family neurotrophin receptors include not only full-length receptor tyrosine kinases but also truncated isoforms that lack kinase activity. Although the truncated isoforms were initially believed to act mainly by inhibiting the function of the full-length isoforms, distinct functions have begun to emerge. Esteban et al. used a yeast two-hybrid system to screen mouse brain cDNA for proteins that interact with a unique C-terminal fragment of TrkCT1, the truncated TrkC isoform, and identified the scaffolding protein tamalin. This interaction was confirmed in an in vitro binding assay. In situ hybridization indicated that expression of TrkCT1 and tamalin in mouse brain overlapped; moreover, when the two proteins were transfected into human embryonic kidney (HEK) 293 cells, immunofluorescence analysis indicated that they colocalized. Neurotrophin-3 (NT3) enhanced coimmunoprecipitation of TrkCT1 and tamalin from HEK293 cells and promoted colocalization of the endogenous proteins in cultured mouse hippocampal neurons. Tamalin forms a complex with the guanine nucleotide exchange factor ARNO (for Arf nucleotide-binding site opener) and, in a HEK293 line with endogenous tamalin that was stably transfected with TrkCT1, NT3 promoted translocation of the guanosine triphosphatase (GTPase) Arf6 to actin-rich membrane ruffles. Arf6 translocation was blocked by tamalin dominant negatives and enhanced by tamalin overexpression. Moreover, NT3-dependent membrane ruffling (monitored by actin polymerization) was blocked by a mutant form of ARNO that cannot promote Arf6 GTP exchange. NT3 stimulated Rac1 activation in HEK293 cells transfected with TrkCT1 and tamalin, and this was blocked by a dominant-negative form of Arf6. Thus, these data define a signaling pathway mediated through the truncated TrkCT1 receptor in which NT3 activates Arf6 and Rac1.
P. F. Esteban, H.-Y. Yoon, J. Becker, S. G. Dorsey, P. Caprari, M. E. Palko, V. Coppola, H. U. Saragovi, P. A. Randazzo, L. Tessarollo, A kinase-deficient TrkC receptor isoform activates Arf6-Rac1 signaling through the scaffold protein tamalin. J. Cell Biol. 173, 291-299 (2006). [Abstract] [Full Text]