The unfolding of substrates and ubiquitin-independent protein degradation by proteasomes

Abstract

26S proteasomes are composed of a 20S proteolytic core and two ATPase-containing 19S regulatory particles. To clarify the role of these ATPases in proteolysis, we studied the PAN complex, the archaeal homolog of the 19S ATPases. When ATP is present, PAN stimulates protein degradation by archaeal 20S proteasomes. PAN is a molecular chaperone that catalyzes the ATP-dependent unfolding of globular proteins. If 20S proteasomes are present, this unfoldase activity is linked to degradation. Thus PAN, and presumably the 26S ATPases, unfold substrates and facilitate their entry into the 20S particle. 26S proteasomes preferentially degrade ubiquitinated proteins. However, we found that calmodulin (CaM) and troponin C are degraded by 26S proteasomes without ubiquitination. Ca(2+)-free native CaM and in vitro 'aged' CaM are degraded faster than the Ca(2+)-bound form. Ubiquitination of CaM does not enhance its degradation. Degradation of ovalbumin normally requires ubiquitination, but can occur without ubiquitination if ovalbumin is denatured. The degradation of these proteins still requires ATP and the 19S particle. Thus, ubiquitin-independent degradation by 26S proteasomes may be more important than generally assumed.