Advances in understanding the molecular mechanisms underlying the ultrastructural organization of bacterial cells have blurred the traditional boundaries used to distinguish eukaryotic cells from their prokaryotic counterparts. Most significant amongst these has been the demonstration that prokaryotic organisms contain functional and structural homologs of eukaryotic tubulin, actin and intermediate filaments. These cytoskeletal proteins are essential for cell division, cell shape maintenance and chromosome segregation in a variety of prokaryotic species and hold promise as targets for the discovery of new antimicrobial agents. Recently, we showed the bacterial actin homolog, MamK, is required for the subcellular organization of the magnetosome organelles of magnetotactic bacteria. These membranous organelles, used by the organism to produce nanometer-sized magnetic crystals, are organized as chains within the cell and surrounded by a distinct cytoskeletal network of filaments. In the absence of MamK this network of filaments fails to form and the magnetosomes are dispersed throughout the cell. Phylogenetic analysis shows that MamK is found outside of the magnetotactic bacteria and that it forms a distinct branch of the bacterial actin like proteins. Since it is not essential for survival and has a distinct localization within the cell it may serve as a tractable system for in-depth analysis of bacterial actin-like proteins in general. We have the following goals for this proposal. 1) Define the biochemical and biophysical properties of MamK in vitro. 2) Identify functional domains on the surface of MamK through a global mutagenesis strategy followed by a series of in vivo analyses. 3) Identify and define the function of MamK-interacting proteins. 4) Determine the genes involved in MamK assembly and magnetosome membrane formation through a genetic analysis of the magnetosome island, a large genomic region containing most of the known magnetosome genes. In addition to their relevance to bacterial actin homologs in general these findings will provide insights into the process of organelle formation in bacteria.