Sm-like protein Hfq: Composition of the native complex, modifications, and interactions

The bacterial Sm-like protein Hfq has been linked functionally to reactions that involve RNA; however, its explicit role and primary cellular localization remain elusive. We carried out a detailed biochemical characterization of native Escherichia coli Hfq obtained through methods that preserve its posttranslational modifications. ESI-MS analyses indicate modifications in 2-3 subunits/hexamer with a molecular mass matching that of an oxidized C:18 lipid. We show that the majority of cellular Hfq cannot be extracted without detergents and that purified Hfq can be retained on hydrophobic matrices. Analyses of purified Hfq and the native Hfq complexes observed in whole-cell E. coli extracts indicate the existence of dodecameric assemblies likely stabilized by interlocking C-terminal polypeptides originating from separate Hfq hexamers and/or accessory nucleic acid. We demonstrate that cellular Hfq is redistributed between transcription complexes and an insoluble fraction that includes protein complexes harboring polynucleotide phosphorylase (PNP). This distribution pattern is consistent with a function at the interface of the apparatuses responsible for synthesis and degradation of RNA. Taken together with the results of prior studies, these results suggest that Hfq could function as an anchor/coupling factor responsible for de-solubilization of RNA and its tethering to the degradosome complex.