Regulation and Kinase Activity of the Trk Family of Receptor Tyrosine Kinases

The tropomyosin-related kinase (Trk) family consists of three receptor tyrosine kinases (RTKs) called TrkA, TrkB, and TrkC. These RTKs are regulated by the neurotrophins, a class of secreted growth factors responsible for the development and function of neurons. Given the high homology between the Trks and their use of overlapping signaling pathways, the key question to be addressed is how the activation of the different Trks can lead to distinct cellular outcomes. To this end, I first sought to determine the mechanism of autoregulation for the Trk tyrosine kinase domain (TKD). The Trk TKDs are members of the insulin receptor kinase (IRK) superfamily and recent data suggest that the IRK family displays a wide array of autoinhibitory mechanisms. To determine where TrkA and the closely related Ror2 TKD (from an unconventional Wnt receptor) lie in this spectrum, we determined the crystal structures of the kinase domains of these RTKs. In both cases, the conformation of the activation loop resembles the IRK activation loop conformation, with subtle but notable differences in the case of Ror2. These findings aid in understanding the range of autoinhibitory mechanisms of the IRK family, in addition to providing a foundation for deciphering consequences of TKD mutations in this family. I also observed crystallographic dimers of the inactive TrkA TKD that resemble those seen for other RTK TKDs - which may aid in understanding the reported pre-formed inactive TrkA dimers observed in cells. To understand the molecular basis for differences in signaling specificity of the Trk receptors, I investigated whether the TrkA and TrkB TKDs differ in their intrinsic kinase activities. I show that the TrkA TKD autophosphorylates itself faster than its TrkB counterpart. However, this difference of autophosphorylation is not due to a difference in kinase activity per se. Rather, my data indicate that the difference in autophosphorylation may arise because of self-association of the TrkA TKD that does not occur with TrkB. My work sheds light on potential differences between TrkA and TrkB signaling, as well as providing a quantitative understanding of Trk TKD activation, which is useful for effective and selective inhibitor design