Structural determination of the two distinct PIKK complexes TORC1 and TORC2

Cell growth is a fundamental process, which has to be tightly regulated. Central kinases involved in growth regulation are the TOR proteins. The TOR proteins are conserved from yeast to man and act in two different complexes: TORC1 and TORC2. TORC1 is specifically inhibited by the macrcocyclic lactone rapamycin, which binds to the proline isomerase FKBP12 and subsequently inhibits TORC1. Why TORC1 activity can be inhibited by the rapamycin-FKBP12 complex is not known. TOR proteins are atypical protein kinases, which show a higher sequence similarity to phospholipid kinases than to classical eukaryotic protein kinases. This classifies the TOR kinases as members of the phosphatidylinositol-3 kinase-related protein kinases (PIKK) family. It is still unknown why these Ser/Thr kinases resemble lipid kinases.

We presently attempt to produce soluble and stable TORC1 and TORC2 from budding yeast for cryo-electron microscopy. The long-term goal will be the crystallization of the complexes for diffraction studies. To gain insight into the regulation of TOR kinases we are attempting to solve the atomic structures. Deregulation of the TOR signaling cascade is implicated in many severe diseases. Regarding the impact of TOR signaling in various diseases, the structures will be useful for rational drug design.