Certain cellular stimuli, such as heat shock, can lead to enhanced rates of degradation of proteins by the ubiquitin-proteasome pathway. Peng et al. have identified one potential source of regulation of global degradation rates in plants. The 26S proteasome is composed of a 20S catalytic core and two 19S regulatory particles, which are important for substrate recognition and preliminary processing. Peng et al. identified another subcomplex, called the PR500 complex (for proteasome-related 500 kD complex), that contains a subset of the proteins found in the complete 19S regulatory complex. Furthermore, in response to heat shock or treatment of seedlings with the arginine analog, canavanine, the amount of the PR500 complex decreased, although the levels of the proteins in the complex remained the same. This suggests that the PR500 complex was incorporated into another structure, such as the 19S regulatory subunit. A link was established between the COP9 signalosome, a multiprotein complex similar in structure to the 19S regulatory complex, and PR500, which is important for regulating photomorphogenesis and protein degradation in Arabidopsis. In Arabidopsis mutants of the COP9 signalosome, the PR500 complex was absent or severely decreased. The authors propose several models for the function of the PR500 complex. The incorporation of the PR500 complex into the 19S subunit may allow for an altered function of the proteasome during conditions of cellular stress.
Z. Peng, J. M. Staub, G. Serino, S. F. Kwok, J. Kurepa, B. D. Bruce, R. D. Vierstra, N. Wei, X.-W. Deng, The cellular level of PR500, a protein complex related to the 19S regulatory particle of the proteasome, is regulated in response to stresses in plants. Mol. Biol. Cell 12, 383-392 (2001). [Abstract] [Full Text]