Solid-state NMR on a large multidomain integral membrane protein: the outer membrane protein assembly factor BamA

Multidomain proteins constitute a large part of prokaryotic and eukaryotic proteomes and play fundamental roles in various physiological processes. However, their structural characterization is challenging because of their large size and intrinsic flexibility. We show here that motional-filtered high-resolution solid-state NMR (ssNMR) experiments allow for the observation and structural analysis of very large multidomain membrane proteins that are characterized by different motional time scales. This approach was used to probe the folding of the 790-residue membrane protein BamA, which is the core component of the Escherichia coli outer membrane protein assembly machinery. A combination of dipolar- and scalar-based two-dimensional ssNMR experiments applied to two uniformly 13C,15N-labeled BamA variants revealed characteristic secondary structure elements and distinct dynamics within the BamA transmembrane protein segment and the periplasmic POTRA domains. This approach hence provides a general strategy for collecting atomic-scale structural information on multidomain (membrane) proteins in a native-like environment.