A sterile alpha-motif domain in NafY targets apo-NifDK for iron-molybdenum cofactor delivery via a tethered domain.

NafY participates in the final steps of nitrogenase maturation, having a dual role as iron-molybdenum cofactor (FeMo-co) carrier and as chaperone to the FeMo-co-deficient apo-NifDK (apo-dinitrogenase). NafY contains an N-terminal domain of unknown function (n-NafY) and a C-terminal domain (core-NafY) necessary for FeMo-co binding. We show here that n-NafY and core-NafY have very weak interactions in intact NafY. The NMR structure of n-NafY reveals that it belongs to the sterile α-motif (SAM) family of domains, which are frequently involved in protein-protein interactions. The presence of a SAM domain in NafY was unexpected and could not be inferred from its amino acid sequence. Although SAM domains are very commonly found in eukaryotic proteins, they have rarely been identified in prokaryotes. The n-NafY SAM domain binds apo-NifDK. As opposed to full-length NafY, n-NafY impaired FeMo-co insertion when present in molar excess relative to FeMo-co and apo-NifDK. The implications of these observations are discussed to offer a plausible mechanism of FeMo-co insertion. NafY domain structure, molecular tumbling, and interdomain motion, as well as NafY interaction with apo-NifDK are consistent with the function of NafY in FeMo-co delivery to apo-NifDK.