Parkin-mediated ubiquitination regulates phospholipase C-γ1

Mutations in parkin cause autosomal recessive forms of Parkinson's disease (PD), with an early age of onset and similar pathological phenotype to the idiopathic disease. Parkin has been identified as an E3 ubiquitin ligase that mediates different types of ubiquitination, which has made the search for substrates an intriguing possibility to identify pathological mechanisms linked to PD. In this study, we present PLC gamma 1 as a novel substrate for parkin. This association was found in non-transfected human neuroblastoma SH-SY5Y cells as well as in stable cell lines expressing parkin WT and familial mutants R42P and G328E. Analysis of cortical, striatal and nigral human brain homogenates revealed that the interaction between parkin and PLC gamma 1 is consistent throughout these regions, suggesting that the interaction is likely to have a physiological relevance for humans. Unlike many of the previously identified substrates, we could also show that the steady-state levels of PLC gamma 1 is significantly higher in parkin KO mice and lower in parkin WT human neuroblastoma cells, suggesting that parkin ubiquitination of PLC gamma 1 is required for proteasomal degradation. In line with this idea, we show that the ability to ubiquitinate PLC gamma 1 in vitro differs significantly between WT and familial mutant parkin. In this study, we demonstrate that parkin interacts with PLC gamma 1, affecting PLC gamma 1 steady state protein levels in human and murine models with manipulated parkin function and expression levels. This finding could be of relevance for finding novel pathogenic mechanisms leading to PD.