A Magnetosome-specific GTPase from the Magnetic Bacterium Magnetospirillum magneticum AMB-1

Abstract

Magnetic bacteria produce intracellular vesicles that envelope single domain magnetite crystals. Although many proteins are present in this intracellular vesicle membrane, five are specific to this membrane. A 16-kDa protein, designated Mmsl6, is the most abundant of the magnetosome-specific proteins, and to establish its function we cloned and sequenced its gene from Magnetospirillum magneticum AMB-1. This was achieved by determination of the N-terminal amino acid sequence of the protein following two dimensional polyacrylamide gel electrophoresis, and sequencing of the gene was performed by gene walking using anchored polymerase chain reaction. Mmsl6 contains a putative ATP/GTP binding motif (P-loop). Recombinant Mmsl6 with a hemagglutinin tag, was expressed in Escherichia coli and purified. Recombinant Mmsl6 protein could bind GTP and showed GTPase activity. GTP was the preferred substrate for Mms16-catalyzed nucleotide triphosphate hydrolysis. These results suggest that a novel protein specifically localized on the magnetic particle membrane, Mmsl6, is a GTPase. Mmsl6 protein showed similar characteristics to small GTPases involved in the formation of intracellular vesicles. Furthermore, addition of the GTPase inhibitor AIF4- also inhibited magnetic particle synthesis, suggesting that GTPase is required for magnetic particles synthesis.