Benaroudj N, Tarcsa E, Cascio P, Goldberg A L
Harvard Medical School, Department of Cell Biology, Boston, MA 02115, USA.
Biochimie. 2001 Mar-Apr;83(3-4):311-8. doi: 10.1016/s0300-9084(01)01244-5.
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.
26S蛋白酶体由一个20S蛋白水解核心和两个含ATP酶的19S调节颗粒组成。为了阐明这些ATP酶在蛋白水解中的作用，我们研究了PAN复合物，它是19S ATP酶的古菌同源物。当有ATP存在时，PAN会刺激古菌20S蛋白酶体的蛋白质降解。PAN是一种分子伴侣，可催化球状蛋白质的ATP依赖性解折叠。如果存在20S蛋白酶体，这种解折叠酶活性与降解相关。因此，PAN以及推测的26S ATP酶会使底物解折叠并促进其进入20S颗粒。26S蛋白酶体优先降解泛素化蛋白质。然而，我们发现钙调蛋白（CaM）和肌钙蛋白C可被26S蛋白酶体在未泛素化的情况下降解。无Ca（2+）的天然CaM和体外“老化”的CaM比与Ca（2+）结合的形式降解得更快。CaM的泛素化不会增强其降解。卵清蛋白的降解通常需要泛素化，但如果卵清蛋白变性，则可以在没有泛素化的情况下发生。这些蛋白质的降解仍然需要ATP和19S颗粒。因此，26S蛋白酶体的非泛素依赖性降解可能比一般认为的更为重要。
