The major magnetosome proteins MamGFDC are not essential for magnetite biomineralization in Magnetospirillum gryphiswaldense but regulate the size of magnetosome crystals

Magnetospirillum gryphiswaldense and related magnetotactic bacteria form magnetosomes, which are membrane-enclosed organelles containing crystals of magnetite (Fe₃O₄) that cause the cells to orient in magnetic fields. The characteristic sizes, morphologies, and patterns of alignment of magnetite crystals are controlled by vesicles formed of the magnetosome membrane (MM), which contains a number of specific proteins whose precise roles in magnetosome formation have remained largely elusive. Here, we report on a functional analysis of the small hydrophobic MamGFDC proteins, which altogether account for nearly 35% of all proteins associated with the MM. Although their high levels of abundance and conservation among magnetotactic bacteria had suggested a major role in magnetosome formation, we found that the MamGFDC proteins are not essential for biomineralization, as the deletion of neither mamC, encoding the most abundant magnetosome protein, nor the entire mamGFDC operon abolished the formation of magnetite crystals. However, cells lacking mamGFDC produced crystals that were only 75% of the wild-type size and were less regular than wild-type crystals with respect to morphology and chain-like organization. The inhibition of crystal formation could not be eliminated by increased iron concentrations. The growth of mutant crystals apparently was not spatially constrained by the sizes of MM vesicles, as cells lacking mamGFDC formed vesicles with sizes and shapes nearly identical to those formed by wild-type cells. However, the formation of wild-type-size magnetite crystals could be gradually restored by in-trans complementation with one, two, and three genes of the mamGFDC operon, regardless of the combination, whereas the expression of all four genes resulted in crystals exceeding the wild-type size. Our data suggest that the MamGFDC proteins have partially redundant functions and, in a cumulative manner, control the growth of magnetite crystals by an as-yet-unknown mechanism.