title: Enabling cell-cell communication via nanopore formation: structure, function and localization of the unique cell wall amidase AmiC2 of Nostoc punctiforme.
abstract: To orchestrate a complex life style in changing environments, the filamentous cyanobacterium Nostoc punctiforme facilitates communication between neighboring cells through septal junction complexes. This is achieved by nanopores that perforate the peptidoglycan (PGN) layer and traverse the cell septa. The N-acetylmuramoyl-l-alanine amidase AmiC2 (Npun_F1846; EC 3.5.1.28) in N. punctiforme generates arrays of such nanopores in the septal PGN, in contrast to homologous amidases that mediate daughter cell separation after cell division in unicellular bacteria. Nanopore formation is therefore a novel property of AmiC homologs. Immunofluorescence shows that native AmiC2 localizes to the maturing septum. The high-resolution crystal structure (1.12 Å) of its catalytic domain (AmiC2-cat) differs significantly from known structures of cell splitting and PGN recycling amidases. A wide and shallow binding cavity allows easy access of the substrate to the active site, which harbors an essential zinc ion. AmiC2-cat exhibits strong hydrolytic activity in vitro. A single point mutation of a conserved glutamate near the zinc ion results in total loss of activity, whereas zinc removal leads to instability of AmiC2-cat. An inhibitory α-helix, as found in the Escherichia coli AmiC(E. coli) structure, is absent. Taken together, our data provide insight into the cell-biological, biochemical and structural properties of an unusual cell wall lytic enzyme that generates nanopores for cell-cell communication in multicellular cyanobacteria. The novel structural features of the catalytic domain and the unique biological function of AmiC2 hint at mechanisms of action and regulation that are distinct from other amidases.
  
  
  METHODS
  The AmiC2-cat structure has been deposited in the Protein Data Bank under accession number 5EMI.
authors.affiliation: Interfaculty Institute of Biochemistry, University of Tübingen, Germany.
authors.forename: Felix M
authors.initials: FM
authors.lastname: Büttner
authors.affiliation: Interfaculty Institute for Microbiology and Infection Medicine, Department of Organismic Interactions, University of Tübingen, Germany.
authors.forename: Katharina
authors.initials: K
authors.lastname: Faulhaber
authors.affiliation: Interfaculty Institute for Microbiology and Infection Medicine, Department of Organismic Interactions, University of Tübingen, Germany.
authors.forename: Karl
authors.initials: K
authors.lastname: Forchhammer
authors.affiliation: Interfaculty Institute for Microbiology and Infection Medicine, Department of Organismic Interactions, University of Tübingen, Germany.
authors.forename: Iris
authors.initials: I
authors.lastname: Maldener
authors.affiliation: Interfaculty Institute of Biochemistry, University of Tübingen, Germany.|Department of Pediatrics, Vanderbilt University School of Medicine, Nashville, TN, USA.
authors.forename: Thilo
authors.initials: T
authors.lastname: Stehle
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites / Q000235:genetics; D055162:Biocatalysis; D020134:Catalytic Domain; D002473:Cell Wall / Q000201:enzymology*; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D008856:Microscopy, Fluorescence; D008958:Models, Molecular; D009154:Mutation; D009238:N-Acetylmuramoyl-L-alanine Amidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D058608:Nanopores; D046937:Nostoc / Q000166:cytology / Q000201:enzymology* / Q000235:genetics; D010457:Peptidoglycan / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D010457:Peptidoglycan; D009238:N-Acetylmuramoyl-L-alanine Amidase
keywords: AmiC; N-acetylmuramoyl-l-alanine amidase; bacterial cell wall; cell-cell communication; multicellular cyanobacteria