Significance: We describe a class of protein–protein interactions mediated by the HORMA (Hop1, Rev7, Mad2) dimerization interface of Rev7, a multitasking scaffolding protein involved in translesion synthesis (TLS), repair of double-strand breaks, and mitosis. Biochemical and structural analyses of Rev7 dimerization reveal an unexpected architecture of the Rev1/Polζ TLS complex, which plays a central role in replication of damaged DNA, and describe the mechanism of Rev7 interactions with HORMA proteins from other pathways. Assays in Rev7 ^−/− cells complemented with mutant Rev7 provide evidence that protein–protein interactions mediated by the Rev7 HORMA interface are important for the DNA damage response. These results contribute to the structural biology of DNA replication and repair and to understanding of the important class of HORMA proteins.

Protein folding is a highly complex process proceeding through a number of disordered and partially folded nonnative states with various degrees of structural organization. These transiently and sparsely populated species on the protein folding energy landscape play crucial roles in driving folding toward the native conformation, yet some of these nonnative states may also serve as precursors for protein misfolding and aggregation associated with a range of devastating diseases, including neuro-degeneration, diabetes and cancer. Therefore, in vivo protein folding is often reshaped co- and post-translationally through interactions with the ribosome, molecular chaperones and/or other cellular components. Owing to developments in instrumentation and methodology, solution NMR spectroscopy has emerged as the central experimental approach for the detailed characterization of the complex protein folding processes in vitro and in vivo. NMR relaxation dispersion and saturation transfer methods provide the means for a detailed characterization of protein folding kinetics and thermodynamics under native-like conditions, as well as modeling high-resolution structures of weakly populated short-lived conformational states on the protein folding energy landscape. Continuing development of isotope labeling strategies and NMR methods to probe high molecular weight protein assemblies, along with advances of in-cell NMR, have recently allowed protein folding to be studied in the context of ribosome-nascent chain complexes and molecular chaperones, and even inside living cells. Here we review solution NMR approaches to investigate the protein folding energy landscape, and discuss selected applications of NMR methodology to studying protein folding in vitro and in vivo. Together, these examples highlight a vast potential of solution NMR in providing atomistic insights into molecular mechanisms of protein folding and homeostasis in health and disease.