Required for assembly of magnetosome chains (PubMed:16299495). Regulates the dynamic behavior of MamK filaments (By similarity). May connect magnetosomes to MamK filaments (Probable). Moves from the cell poles towards midcell; movement does not depend on the treadmilling ability of MamK, suggesting MamJ associates and disassociates continuously from the MamK filament (PubMed:27733152). Forms homooligomers (Probable) (PubMed:17601786). Interacts with MamK (PubMed:17601786). Tagged protein extends from one pole to the other, and is longer than the magnetosome chain (PubMed:16299495). Correct subcellular location requires MamK (PubMed:20487281). Part of the probable 17 gene mamAB operon. The gene is quite polymorphic, the central region is subject to recombination between internal Glu-Pro-rich repeats (Probable). The polymorphic central region is not required for function, small segments in the N- and C-terminus are however required (PubMed:17601786). Identified by N-terminal sequencing of a protein that is about 96 kDa in size (PubMed:14766587). The protein runs anomalously on protein gels (PubMed:17601786). Single gene deletion has compact clusters of magnetosomes instead of magnetosome chains; precursor magnetosome vesicles and vesicles containing immature crystals are throughout the cytoplasm and no longer associated with filaments. Cells have substantially decreased magnetic orientation (PubMed:16299495, PubMed:17601786). No change in location of MamK (PubMed:17601786). Single deletion slows recovery of MamK dynamics following photobleaching (PubMed:27733152). Deletion of 3 consecutive genes (mamJ, mamK, mamL) leads to cells devoid of magnetosomes or a magnetic response (PubMed:22043287). Deletion of approximately 80 kb of DNA, including this operon, leads to cells that are non-magnetic, lack internal membrane systems, grow poorly, have reduced mobility and take-up and accumulate iron poorly (PubMed:13129949). The gene is quite polymorphic, the central region is subject to recombination between similar Glu- and Pro-rich repeats. This bacteria makes up to 60 cubo-octahedral magnetosomes of about 45 nm in diameter which contain membrane-bound crystals of magnetite (Fe(3)O(4)). Expression of just the minimal mamAB gene cluster (MGMSRv2__2365 to MGMSRv2__2381), including this gene, is sufficient to form a minimal magnetosome chain with small magnetite particles. Belongs to the magnetosome MamJ protein family.