Structural insights into the membrane microdomain organization by SPFH family proteins.

The lateral segregation of membrane constituents into functional microdomains, conceptually known as lipid raft, is a universal organization principle for cellular membranes in both prokaryotes and eukaryotes. The widespread Stomatin, Prohibitin, Flotillin, and HflK/C (SPFH) family proteins are enriched in functional membrane microdomains at various subcellular locations, and therefore were hypothesized to play a scaffolding role in microdomain formation. In addition, many SPFH proteins are also implicated in highly specific processes occurring on the membrane. However, none of these functions is understood at the molecular level. Here we report the structure of a supramolecular complex that is isolated from bacterial membrane microdomains and contains two SPFH proteins (HflK and HflC) and a membrane-anchored AAA+ protease FtsH. HflK and HflC form a circular 24-mer assembly, featuring a laterally segregated membrane microdomain (20 nm in diameter) bordered by transmembrane domains of HflK/C and a completely sealed periplasmic vault. Four FtsH hexamers are embedded inside this microdomain through interactions with the inner surface of the vault. These observations provide a mechanistic explanation for the role of HflK/C and their mitochondrial homologs prohibitins in regulating membrane-bound AAA+ proteases, and suggest a general model for the organization and functionalization of membrane microdomains by SPFH proteins.

The Role of Iron Competition in the Antagonistic Action of the Rice Endophyte Streptomyces sporocinereus OsiSh-2 Against the Pathogen Magnaporthe oryzae.

Rice blast caused by Magnaporthe oryzae severely impacts global rice yield stability. The rice endophyte Streptomyces sporocinereus OsiSh-2, with strong antagonistic activity towards M. oryzae, has been reported in our previous study. To decipher the model of the antagonistic action of OsiSh-2 towards M. oryzae, we compared the iron-capturing abilities of these two strains. The cultivation of OsiSh-2 and a M. oryzae strain under iron-rich and iron-starved conditions showed that M. oryzae depended more on iron supplementation for growth and development than did OsiSh-2. Genomic analysis of the S. sporocinereus and M. oryzae species strains revealed that they might possess different iron acquisition strategies. The actinobacterium OsiSh-2 is likely to favor siderophore utilization compared to the fungus M. oryzae. In addition, protein annotations found that OsiSh-2 contains the highest number of the siderophore biosynthetic gene clusters among the 13 endophytic actinomycete strains and 13 antifungal actinomycete strains that we compared, indicating the prominent siderophore production potential of OsiSh-2. Additionally, we verified that OsiSh-2 could excrete considerably more siderophores than Guy11 under iron-restricted conditions and displayed greater Fe3+-reducing activity during iron-supplemental conditions. Measurements of the iron mobilization between the antagonistic OsiSh-2 and Guy11 showed that the iron concentration is higher around OsiSh-2 than around Guy11. In addition, adding iron near OsiSh-2 could decrease the antagonism of OsiSh-2 towards Guy11. Our study revealed that the antagonistic capacity displayed by OsiSh-2 towards M. oryzae was related to the competition for iron. The highly efficient iron acquisition system of OsiSh-2 may offer valuable insight for the biocontrol of rice blast.