Parkin mediates nonclassical, proteasomal-independent ubiquitination of synphilin-1: Implications for Lewy body formation

It is widely accepted that the familial Parkinson's disease ( PD)- linked gene product, parkin, functions as a ubiquitin ligase involved in protein turnover via the ubiquitin - proteasome system. Substrates ubiquitinated by parkin are hence thought to be destined for proteasomal degradation. Because we demonstrated previously that parkin interacts with and ubiquitinates synphilin- 1, we initially expected synphilin- 1 degradation to be enhanced in the presence of parkin. Contrary to our expectation, we found that synphilin- 1 is normally ubiquitinated by parkin in a nonclassical, proteasomal- independent manner that involves lysine 63 ( K63)- linked polyubiquitin chain formation. Parkin- mediated degradation of synphilin- 1 occurs appreciably only at an unusually high parkin to synphilin- 1 expression ratio or when primed for lysine 48 ( K48)- linked ubiquitination. In addition we found that parkin- mediated ubiquitination of proteins within Lewy- body- like inclusions formed by the coexpression of synphilin- 1, alpha- synuclein, and parkin occurs predominantly via K63 linkages and that the formation of these inclusions is enhanced by K63- linked ubiquitination. Our results suggest that parkin is a dual- function ubiquitin ligase and that K63- linked ubiquitination of synphilin- 1 by parkinmaybe involved in the formation of Lewy body inclusions associated with PD.