@article {Siponen1319, author = {Siponen, Marina~I. and Adryanczyk, G{\'e}raldine and Ginet, Nicolas and Arnoux, Pascal and Pignol, David}, title = {Magnetochrome: a c-type cytochrome domain specific to magnetotatic bacteria}, volume = {40}, number = {6}, pages = {1319--1323}, year = {2012}, doi = {10.1042/BST20120104}, publisher = {Portland Press Limited}, abstract = {Magnetotactic bacteria consist of a group of taxonomically, physiologically and morphologically diverse prokaryotes, with the singular ability to align with geomagnetic field lines, a phenomenon referred to as magnetotaxis. This magnetotactic property is due to the presence of iron-rich crystals embedded in lipidic vesicles forming an organelle called the magnetosome. Magnetosomes are composed of single-magnetic-domain nanocrystals of magnetite (Fe3O4) or greigite (Fe3S4) embedded in biological membranes, thereby forming a prokaryotic organelle. Four specific steps are described in this organelle formation: (i) membrane specialization, (ii) iron acquisition, (iii) magnetite (or greigite) biocrystallization, and (iv) magnetosome alignment. The formation of these magnetic crystals is a genetically controlled process, which is governed by enzyme-catalysed processes. On the basis of protein sequence analysis of genes known to be involved in magnetosome formation in Magnetospirillum magneticum AMB-1, we have identified a subset of three membrane-associated or periplasmic proteins containing a double cytochrome c signature motif CXXCH: MamE, MamP and MamT. The presence of these proteins suggests the existence of an electron-transport chain inside the magnetosome, contributing to the process of biocrystallization. We have performed heterologous expression in E. coli of the cytochrome c motif-containing domains of MamE, MamP and MamT. Initial biophysical characterization has confirmed that MamE, MamP and MamT are indeed c-type cytochromes. Furthermore, determination of redox potentials for this new family of c-type cytochromes reveals midpoint potentials of -76 and -32 mV for MamP and MamE respectively.Abbreviations: MAI, magnetosome genetic island}, issn = {0300-5127}, URL = {http://www.biochemsoctrans.org/content/40/6/1319}, eprint = {http://www.biochemsoctrans.org/content/40/6/1319.full.pdf}, journal = {Biochemical Society Transactions} }