C-terminal truncation and Parkinson's disease-associated mutations down-regulate the protein serine/threonine kinase activity of PTEN-induced kinase-1

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Abstract
The Parkinson's disease (PD) causative PINK1 gene encodes a mitochondrial protein kinase called PTEN-induced kinase 1 (PINK1). The autosomal recessive pattern of inheritance of PINK1 mutations suggests that PINK1 is neuroprotective and therefore loss of PINK1 function causes PD. Indeed, overexpression of PINK1 protects neuroblastoma cells from undergoing neurotoxin-induced apoptosis. As a protein kinase, PINK1 presumably exerts its neuroprotective effect by phosphorylating specific mitochondrial proteins and in turn modulating their functions. Towards elucidation of the neuroprotective mechanism of PINK1, we employed the baculovirus-infected insect cell system to express the recombinant protein consisting of the PINK1 kinase domain either alone [PINK1(KD)] or with the PINK1 C-terminal tail [PINK1(KD+T)]. Both recombinant enzymes preferentially phosphorylate the artificial substrate histone H1 exclusively at serine and threonine residues, demonstrating that PINK1 is indeed a protein serine/threonine kinase. Introduction of the PD-associated mutations, G386A and G409V significantly reduces PINK1(KD) kinase activity. Since Gly-386 and Gly-409 reside in the conserved activation segment of the kinase domain, the results suggest that the activation segment is a regulatory switch governing PINK1 kinase activity. We also demonstrate that PINK1(KD+T) is ∼6-fold more active than PINK1(KD). Thus, in addition to the activation segment, the C-terminal tail also contains regulatory motifs capable of governing PINK1 kinase activity. Finally, the availability of active recombinant PINK1 proteins permits future studies to search for mitochondrial proteins that are preferentially phosphorylated by PINK1. As these proteins are likely physiological substrates of PINK1, their identification will shed light on the mechanism of pathogenesis of PD.