A protein-protein interaction in magnetosomes: TPR protein MamA interacts with an Mms6 protein

• New advance in the 20-year history of MamA.
• Two types of Mms6 exist in the magnetosome membrane, 14.5-kDa and 6.0-kDa versions.
• Mms6 is the binding partner of a TPR protein (MamA) in magnetosome.
• Describes a new role for Mms6 in magnetotactic bacteria.

Magnetosomes are membrane-enveloped bacterial organelles containing nano-sized magnetic particles, and function as a cellular magnetic sensor, which assist the cells to navigate and swim along the geomagnetic field. Localized with each magnetosome is a suite of proteins involved in the synthesis, maintenance and functionalization of the organelle, however the detailed molecular organization of the proteins in magnetosomes is unresolved. MamA is one of the most abundant magnetosome-associated proteins and is anchored to the magnetosome vesicles through protein-protein interactions, but the identity of the protein that interacts with MamA is undetermined. In this study, we found that MamA binds to a magnetosome membrane protein Mms6. Two different molecular masses of Mms6, 14.5-kDa and 6.0-kDa, were associated with the magnetosomes. Using affinity chromatography, we identified that the 14.5-kDa Mms6 interacts with MamA, and the interaction was further confirmed by pull-down, immunoprecipitation and size-exclusion chromatography assays. Prior to this, Mms6 was assumed to be strictly involved with biomineralizing magnetite; however, these results suggest that Mms6 has an additional responsibility, binding to MamA.