In cells, many constituents are able to assemble resulting in large macromolecular machineries possessing very specific biological and physiological functions, e.g. ribosome, spliceosome and proteasome. Assembly of such entities is commonly mediated by transient protein factors. SPAG1 is a multidomain protein, known to participate in the assembly of both the inner and outer dynein arms. These arms are required for the function of sensitive and motile cells. Together with RUVBL1, RUVBL2 and PIH1D2, SPAG1 is a key element of R2SP, a protein complex assisting the quaternary assembly of specific protein clients in a tissue-specific manner and associating with heat shock proteins (HSPs) and regulators. In this study, we have investigated the role of TPR domains of SPAG1 in the recruitment of HSP chaperones by combining biochemical assays, ITC, NMR spectroscopy and molecular dynamics (MD) simulations. First, we propose that only two, out of the three TPR domains, are able to recruit the protein chaperones HSP70 and HSP90. We then focused on one of these TPR domains and elucidated its 3D structure using NMR spectroscopy. Relying on an NMR-driven docking approach and MD simulations, we deciphered its binding interface with the C-terminal tails of both HSP70 and HSP90. Finally, we addressed the biological function of SPAG1 and specifically demonstrated that a SPAG1 sub-fragment, containing a putative P-loop motif, cannot efficiently bind and hydrolyze GTP in vitro. Our data challenge the interpretation of SPAG1 possessing GTPase activity. We propose instead that SPAG1 regulates nucleotide hydrolysis activity of the HSP and RUVBL1/2 partners.

Tetratricopeptide Repeat domains, or TPR domains, are protein domains that mediate protein:protein interaction. As they allow contacts between proteins, they are of particular interest in transient steps of the assembly process of macromolecular complexes, such as the ribosome or the dynein arms. In this study, we focused on the first TPR domain of the human SPAG1 protein. SPAG1 is a multidomain protein important for ciliogenesis whose known mutations are linked to the primary ciliary dyskinesia syndrome. It can interact with the chaperones RUVBL1/2, HSP70 and HSP90. Using protein sequence optimization in combination with structural and biophysics approaches, we analyzed, with an atomistic precision, how the C-terminal tails of HSPs bind a variant form of SPAG1-TPR1 that mimics the wildtype domain. We discussed our results in regard of others complex 3D structures with the aim to highlight the motifs in the TPR sequences that could drive the positioning of the HSP peptides. These data could be of importance for the draggability of TPR regulators.

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