The neurodegenerative disease Parkinson’s is typically found in the over 60’s, however around 10 % of cases are autosomal recessive juvenile forms caused by mutations in one of the PARK genes, 15 of which have been annotated to date. PINK1, which encodes a Ser/Thr kinase, was one of the first PARK genes to be identified, and is also one of the most divergent protein kinases in humans. PINK1 triggers the clearance of damaged mitochondria by phosphorylating the small regulatory protein Ubiquitin. Recently, a new conformation of ubiquitin was discovered, termed ubiquitin C-terminally retracted conformation, or Ub-CR. This new ubiquitin conformation could be stabilised by point mutations (Ub TVLN) and it could be shown that it binds to PINK1 with higher affinity than the wild-type form. Moreover, it seems that the Ub-CR conformation is required for efficient phosphorylation by PINK1.
Despite its divergence from other human protein kinases, PINK1 is highly conserved between species leading to the identification of an insect homologue, (Ph)PINK1, which can more easily be expressed in the lab for experimental purposes. In a recent study, scientists were able to determine the crystal structure of PhPINK1 bound to Ub TVLN with the help of a stabilising nanobody. Dr Alexander Schubert, first author of the study, commented “although PINK1 has been extensively studied over the last ten years, it was not possible to crystallise this highly dynamic protein. We therefore had no understanding of how this unusual kinase operates and how it was able to phosphorylate ubiquitin. Ubiquitin, on the other hand, is also a very unusual kinase substrate, as its phosphorylation site (Ser65) is not easily accessible. PINK1 is highly mutated in early-onset Parkinson's disease and with a structure we would be able to understand several patient mutations on a molecular level and might guide the design of Parkinson's disease therapeutics”.
The complex structure helped to understand the molecular basis of more than 40 PINK1 mutations, which may cause detrimental changes that lead to Parkinson’s disease. It is hoped that this information will then enable the design of Parkinson’s disease therapeutics. Alex concluded “having the structure of PINK1 in its active conformation bound to its substrate ubiquitin is a very good starting point for the development of PINK1-activating small molecules. We are quite positive that our structure will guide the design of drugs to treat early-onset Parkinson's disease in the future.” In a wider context, the structure also reveals information about how the kinase enzyme family, which are a major drug target in their own right, interact with their substrates opening doors for improved therapies for many more debilitating diseases.
Structure of PiNK1 in complex with its substrate ubiquitin Alexander F. Schubert, Christina Gladkova, Els Pardon, Jane L. Wagstaff, Stefan M.V. Freund, Jan Steyaert, Sarah L. Maslen and David Komander. Nature http://dx.doi.org/10.1038/nature24645 (2017).