ATP hydrolysis by HSP90

The chaperoning function of HSP90 is coupled to its ATPase activity. Our current understanding of the ATPase mechanism of Hsp90 is based largely on structural and functional studied for the Saccharomyces cerevisiae Hsp90 complexes (Meyer P et al. 2003, 2004; Ali MM et al. 2006; Prodromou C et al. 2000; Prodromou C 2012). The ATPase cycle of human HSP90 is less well understood, however several studies suggest that the underlying enzymatic mechanisms and a set of conformational changes that accompany the ATPase cycle are highly similar in both species (Richter K et al. 2008; Vaughan CK et al. 2009). Once ATP is bound it helps to stabilize the closed ATP lid state, in which the gamma-phosphate of ATP provides a hydrogen bonding that promotes a stable association of the ATP lid with N-terminal domain (NTD) (Ali MM et al. 2006; Prodromou C et al. 2000; Chadli A et al. 2000). The association of ATP with NTD then stimulates structural changes in NTD and in the middle domain that are likely to involve movements of the ATP lid segment within each N-terminal domain that locates over the bound ATP. The movement of the lids exposes surface residues that are subsequently involved in transient dimerization of the N-terminal domains of HSP90 (Ali MM et al. 2006; Prodromou C et al. 2000; Chadli A et al. 2000). Furthermore, the intrachain associations of NTD with the middle domain leads to the active conformation of the catalytic loop of HSP90, which commits the ATP for hydrolysis (Meyer P et al. 2003). The subsequent conformational changes upon ATP binding are regulated by co-chaperone activities. For example, arrangement of the STIP1 domains in the complex seems to prevent the NTDs dimerization of HSP90 monomers and total closure of the HSP90 dimer that is required for an efficient HSP90-mediated ATP hydrolysis (Southworth DR & Agard DA 2011; Alvira S et al. 2014). In addition, client protein binding to HSP90 was found to increase ATPase activity of HSP90 up to 200-fold (McLaughlin SH et al. 2002).<p>After hydrolysis of ATP the ligand-bound steroid hormone receptor (SHR) is released from HSP90 complex. The Reactome module describes ATPase activity of HSP90 in the nucleus, however it is not entirely clear whether cytosolic hormone-bound SHR translocates through the nuclear pores before or after ATP-dependent dissociation from the HSP90 complex.

HSP90的伴侣功能与其ATP酶活性紧密相连。目前我们对Hsp90的ATP酶机制的理解主要基于对酿酒酵母Hsp90复合物的结构和功能研究（Meyer P 等人，2003，2004；Ali MM 等人，2006；Prodromou C 等人，2000；Prodromou C，2012）。尽管人类HSP90的ATP酶周期理解尚不充分，但多项研究表明，两种物种在ATP酶周期背后的酶促机制及其伴随的构象变化具有高度相似性（Richter K 等人，2008；Vaughan CK 等人，2009）。ATP结合后，有助于稳定封闭的ATP盖状态，其中ATP的γ-磷酸提供氢键，促进ATP盖与N端结构域（NTD）的稳定结合（Ali MM 等人，2006；Prodromou C 等人，2000；Chadli A 等人，2000）。ATP与NTD的结合随后刺激NTD和中域的结构变化，这些变化可能涉及每个N端结构域中ATP盖片段的运动，而这些结构域位于结合的ATP上方。盖子的运动暴露了随后参与HSP90 N端结构域瞬态二聚化的表面残基（Ali MM 等人，2006；Prodromou C 等人，2000；Chadli A 等人，2000）。此外，NTD与中域的链内相互作用导致HSP90催化环的活性构象，从而将ATP用于水解（Meyer P 等人，2003）。ATP结合后的后续构象变化受辅助伴侣活性的调控。例如，复合物中STIP1结构域的排列似乎防止了HSP90单体NTDs的二聚化以及HSP90二聚体的完全闭合，这对于有效的HSP90介导的ATP水解至关重要（Southworth DR & Agard DA，2011；Alvira S 等人，2014）。此外，研究发现，客户端蛋白与HSP90的结合可将HSP90的ATP酶活性提高至200倍（McLaughlin SH 等人，2002）。在ATP水解之后，结合配体的类固醇激素受体（SHR）从HSP90复合物中释放。Reactome模块描述了HSP90在细胞核中的ATP酶活性，然而，细胞质中激素结合的SHR在ATP依赖性解离HSP90复合物之前或之后通过核孔是否发生转位，尚不十分明确。