Escherichia coliSPFH membrane microdomain proteins HflKC contribute to aminoglycoside and oxidative stress tolerance

Abstract

Eukaryotic cells segregate many membrane-dependent functions into membrane microdomains also known as lipid rafts. These domains are enriched in polyisoprenoid lipids and in scaffolding proteins belonging to the Stomatin, Prohibitin, Flotillin, and HflK/C (SPFH) protein superfamily, which are also found in prokaryotes. Whereas Gram-positive bacteria were also shown to possess functional membrane microdomains (FMMs) structurally and functionally similar to eukaryotic lipid rafts, little is still known about Gram-negative bacteria FMMs. Escherichia coli K12 possesses 4 SPFH proteins, YqiK, QmcA, HflK, and HflC, previously shown to localize in discrete polar or lateral inner-membrane locations, raising the possibility that E. coli SPFH proteins could contribute to the assembly of inner-membrane FMMs regulating cellular processes. Here we studied the determinants of the native, chromosomal QmcA and HflC cell localization using a domain swap analysis and fluorescent and super-resolution microscopy. We showed that full QmcA and HflC protein is required for achieving their native inner-membrane localization and that impairing the synthesis of cardiolipin and isoprenoid lipids known to associate with FMMs alters QmcA and HflC localization pattern. Finally, using Biolog phenotypic arrays, we showed that a mutant lacking all SPFH genes displayed increased sensitivity to aminoglycosides and oxidative stress. This phenotype is exclusively due to the absence of HflKC and a cross-linking and mass spectrometry analysis showed that YajC, a SecDF translocon accessory protein, interacts with HflC and also contributes to E. coli stress tolerance. Our study therefore provides insights into the function and interactions associated with SPFH proteins in E. coli FMMs.