CHIP (carboxy terminus of Hsc70-interacting
protein) an
E3 ubiquitin ligase that binds to Hsp70 and Hsp90, promotes degradation of several Hsp90-regulated signaling
proteins and disease-causing
proteins containing expanded
glutamine tracts. In
polyglutamine disease models, CHIP has been considered a primary protection factor by promoting degradation of these misfolded
proteins. Here, we show that two CHIP substrates, the
glucocorticoid receptor (GR), a classic Hsp90-regulated signaling
protein, and the expanded
glutamine androgen receptor (AR112Q), are degraded at the same rate in CHIP(-/-) and CHIP(+/+) mouse embryonic fibroblasts
after treatment with the Hsp90 inhibitor
geldanamycin. CHIP(-/-) cytosol has the same ability as CHIP(+/+) cytosol to ubiquitinate purified
neuronal nitric oxide synthase (nNOS), another established CHIP substrate. To determine whether other E3
ubiquitin ligases that bind to Hsp70 (Parkin) or Hsp90 (Mdm2) act on CHIP substrates, each
E3 ligase was co-expressed with the GR, nNOS, AR112Q or Q78
ataxin-3. CHIP lowered the levels of all four
proteins, Parkin acted on nNOS and Q78
ataxin-3 but not on the
steroid receptors, and Mdm2 did not affect any of the co-expressed
proteins. Moreover, both CHIP and Parkin co-localized to aggregates of the expanded
glutamine AR formed in cell culture and in a knock-in mouse model of
spinal and bulbar muscular atrophy. These observations establish that CHIP does not play an exclusive role in regulating the turnover of Hsp90 client signaling
proteins or expanded
glutamine tract
proteins, and show that the Hsp70-dependent
E3 ligase Parkin acts redundantly to CHIP on some substrates.