Smith David M, Benaroudj Nadia, Goldberg Alfred
Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA.
J Struct Biol. 2006 Oct;156(1):72-83. doi: 10.1016/j.jsb.2006.04.012. Epub 2006 May 8.
Protein degradation by 20S proteasomes in vivo requires ATP hydrolysis by associated hexameric AAA ATPase complexes such as PAN in archaea and the homologous ATPases in the eukaryotic 26S proteasome. This review discusses recent insights into their multistep mechanisms and the roles of ATP. We have focused on the PAN complex, which offers many advantages for mechanistic and structural studies over the more complex 26S proteasome. By single-particle EM, PAN resembles a "top-hat" capping the ends of the 20S proteasome and resembles densities in the base of the 19S regulatory complex. The binding of ATP promotes formation of the PAN-20S complex, which induces opening of a gate for substrate entry into the 20S. PAN's C-termini, containing a conserved motif, docks into pockets in the 20S's alpha ring and causes gate opening. Surprisingly, once substrates are unfolded, their translocation into the 20S requires ATP-binding but not hydrolysis and can occur by facilitated diffusion through the ATPase in its ATP-bound form. ATP therefore serves multiple functions in proteolysis and the only step that absolutely requires ATP hydrolysis is the unfolding of globular proteins. The 26S proteasome appears to function by similar mechanisms.
在体内，20S蛋白酶体介导的蛋白质降解需要相关的六聚体AAA型ATP酶复合物水解ATP，如古细菌中的PAN以及真核生物26S蛋白酶体中的同源ATP酶。本综述讨论了对其多步机制和ATP作用的最新见解。我们重点关注了PAN复合物，与更为复杂的26S蛋白酶体相比，它在机制和结构研究方面具有诸多优势。通过单颗粒电子显微镜观察，PAN类似于一顶“礼帽”覆盖在20S蛋白酶体的两端，并且与19S调节复合物底部的密度相似。ATP的结合促进了PAN-20S复合物的形成，该复合物诱导底物进入20S的通道打开。PAN的C末端包含一个保守基序，它对接在20S的α环中的口袋中并导致通道打开。令人惊讶的是，一旦底物被展开，它们向20S的转运需要ATP结合但不需要水解，并且可以通过以ATP结合形式的ATP酶促进扩散来实现。因此，ATP在蛋白水解中发挥多种功能，绝对需要ATP水解的唯一步骤是球状蛋白质的展开。26S蛋白酶体似乎通过类似的机制发挥作用。
