The polyglutamine neurodegenerative protein ataxin-3 binds polyubiquitylated proteins and has ubiquitin protease activity

The ubiquitin-proteasome pathway is critically involved in the pathology of neurodegenerative diseases characterized by protein misfolding and aggregation. Data in the present study suggest that the polyglutamine neurodegenerative disease protein, ataxin-3 (AT3), functions in the ubiquitin-proteasome pathway. AT3 contains an ubiquitin interaction motif (UIM) domain that binds polyubiquitylated proteins with a strong preference for chains containing four or more ubiquitins. Mutating the conserved leucine in the first UIM (L229A) almost totally eliminates binding to polyubiquitin chains while a similar mutation in the second UIM (L249A) also inhibits binding to polyubiquitin chains but to a lesser extent. Both wild-type and pathological AT3 increase cellular levels of a short-lived GFP that is degraded by the ubiquitin-proteasome pathway. AT3 has several properties characteristic of ubiquitin proteases including decreasing polyubiquitylation of ¹²⁵I-lysozyme by removing ubiquitin from polyubiquitin chains, cleaving a ubiquitin protease substrate, and binding the specific ubiquitin protease inhibitor, ubiquitin-aldehyde. Mutating the predicted catalytic cysteine in AT3 inhibits each of these ubiquitin protease activities. The ability to bind and cleave ubiquitylated proteins is consistent with AT3 playing a role in the ubiquitin-proteasome system. This raises the possibility that pathological AT3, which tends to misfold and aggregate, may be exposed to aggregate-prone misfolded/denatured proteins as part of its normal function.