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Journal of Bacteriology, April 2005, p. 2416-2425, Vol. 187, No. 7
0021-9193/05/$08.00+0     doi:10.1128/JB.187.7.2416-2425.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

The Presumptive Magnetosome Protein Mms16 Is a Poly(3-Hydroxybutyrate) Granule-Bound Protein (Phasin) in Magnetospirillum gryphiswaldense

Daniel Schultheiss,¹ René Handrick,^2, Dieter Jendrossek,² Marianne Hanzlik,³ and Dirk Schüler^1*

Max Planck Institute for Marine Microbiology, Bremen,¹ Institute for Microbiology, University Stuttgart, Stuttgart,² Institute for Technical Chemistry, Department of Electron Microscopy, Technical University Munich, Garching, Germany³

Received 19 August 2004/ Accepted 20 December 2004

The Mms16 protein has been previously found to be associated with isolated magnetosomes from two Magnetospirillum strains. A function of this protein as a magnetosome-specific GTPase involved in the formation of intracellular magnetosome membrane vesicles was suggested (Y. Okamura, H. Takeyama, and T. Matsunaga, J. Biol. Chem. 276:48183-48188, 2001). Here we present a study of the Mms16 protein from Magnetospirillum gryphiswaldense to clarify its function. Insertion-duplication mutagenesis of the mms16 gene did not affect the formation of magnetosome particles but resulted in the loss of the ability of M. gryphiswaldense cell extracts to activate poly(3-hydroxybutyrate) (PHB) depolymerization in vitro, which was coincident with loss of the most abundant 16-kDa polypeptide from preparations of PHB granule-bound proteins. The mms16 mutation could be functionally complemented by enhanced yellow fluorescent protein (EYFP) fused to ApdA, which is a PHB granule-bound protein (phasin) in Rhodospirillum rubrum sharing 55% identity to Mms16. Fusions of Mms16 and ApdA to enhanced green fluorescent protein (EGFP) or EYFP were colocalized in vivo with the PHB granules but not with the magnetosome particles after conjugative transfer to M. gryphiswaldense. Although the Mms16-EGFP fusion protein became detectable by Western analysis in all cell fractions upon cell disruption, it was predominantly associated with isolated PHB granules. Contrary to previous suggestions, our results argue against an essential role of Mms16 in magnetosome formation, and the previously observed magnetosome localization is likely an artifact due to unspecific adsorption during preparation. Instead, we conclude that Mms16 in vivo is a PHB granule-bound protein (phasin) and acts in vitro as an activator of PHB hydrolysis by R. rubrum PHB depolymerase PhaZ1. Accordingly, we suggest renaming the Mms16 protein of Magnetospirillum species to ApdA, as in R. rubrum.

Present address: Experimental Radiation Oncology, University of Tübingen, Tübingen, Germany.

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