Substrate recognition and function of the R2TP complex in response to cellular stress

Abstract

The R2TP complex is a HSP90 co-chaperone, which consists of four subunits: PIH1D1, RPAP3, RUVBL1, and RUVBL2. It is involved in the assembly of large protein or protein-RNA complexes such as RNA polymerase, small nucleolar ribonucleoproteins (snoRNPs), phosphatidylinositol 3 kinase-related kinases (PIKKs), and their complexes. While RPAP3 has a HSP90 binding domain and the RUVBLs comprise ATPase activities important for R2TP functions, PIH1D1 contains a PIH-N domain that specifically recognizes phosphorylated substrates of the R2TP complex. In this review we provide an overview of the current knowledge of the R2TP complex with the focus on the recently identified structural and mechanistic features of the R2TP complex functions. We also discuss the way R2TP regulates cellular response to stress caused by low levels of nutrients or by DNA damage and its possible exploitation as a target for anti-cancer therapy.

Keywords: DNA damage response; R2TP complex; cancer; cellular stress; protein folding.

FIGURE 1

(Potential) functions of the R2TP complex. Known functions of the R2TP complex are indicated in blue, potential R2TP functions are indicated in red. SMG1 complex and mTOR stabilization by the R2TP complex requires binding of PIH1D1 with the PIH-N domain to TEL2. Other potential PIH-N domain binding proteins are indicated, but their role for R2TP complex function remains to be studied.

FIGURE 2

HSP90-R2TP complex in yeast and mammals. Proteins homologous between Yeast and mammals are shown in the same color. N indicates the N-terminal domain, C the C-terminal domain and M the middle domain of a protein. Yeast Tah1 contains one TPR domain and therefore two Tah1 molecules are needed to bind the Hsp90 dimer. Tah1 binds Pih1, which in turn binds R2TP target substrates with its N-terminal domain and the Rvbl proteins with its middle domain. In contrast the mammalian R2TP complex binds the HSP90 dimer with one RPAP3 molecule containing two TPR domains. The RPAP3 C-terminal domain connects directly or indirectly with the other components of the complex, namely PIH1D1 the RUVBLs and the prefoldin-like complex components (absent in yeast R2TP).

FIGURE 3

Assembly of multiple-subunit complexes by R2TP. The multiple-subunit complexes in mammalian cells are assembled in several steps: the early phase requires presence of R2TP complex, which may serve to remove assembly inhibitors (I) and/or to load RUVBL1/2 to the premature complexes in ATP dependent manner. RUVBL1/2 helps in the latter stage of assembly forming the mature complexes and either disassociate from or become part of the mature complexes. The substrate (X, Y) specificity is given by presence of phosphorylated sequence (P), recognized by PIH-N domain and possibly by specific interactions with other components of the R2TP complex.