Characterization of the cellular signaling mechanisms of Tribbles pseudokinases

Pseudokinases are members of the protein kinase superfamily that are predicted to be catalytically inactive despite retaining the conserved kinase domain fold. Consequently, pseudokinases have evolved to signal exclusively through non-catalytic mechanisms, such as allostery and scaffolding. Though pseudokinases have been implicated in a wide range of diseases and are known to be involved in critical cellular processes, their lack of catalytic activity has made their functions challenging to study. Members of the Tribbles family of pseudokinases have recently emerged as key regulators of diverse signaling pathways, including the MAPK cascade, proteasome-dependent degradation, the TGF-β/BMP pathway, and the PI3K/Akt pathway. However, the molecular mechanisms underlying Tribbles-mediated signaling are poorly understood. One of the most highly conserved functions of Tribbles pseudokinases is their role in promoting ubiquitination of C/EBP transcription factors by the E3 ubiquitin ligase COP1. Dysregulation of this function has been implicated in acute myeloid leukemia, where overexpression of Trb1 or Trb2 is sufficient to induce leukemogenesis in mice through depletion of C/EBPα. Tribbles pseudokinases are thought to function as scaffolds to facilitate interactions between COP1 and substrates like C/EBPα, but the molecular basis for this function is only beginning to be uncovered. Here, we have investigated the molecular and structural determinants of regulation of COP1 by the pseudokinase Trb1. In doing so, we have uncovered a previously uncharacterized role for Trb1 in promoting COP1 nuclear localization through disrupting a conserved intramolecular regulatory interaction within COP1, and we have identified key structural elements that control COP1 localization. Collectively, these findings will deepen our understanding of the mechanisms underlying regulation of COP1 by Tribbles pseudokinases and broaden our knowledge of the diverse signaling functions of the Tribbles family