Small membranous proteins of the TorE/NapE family, crutches for cognate respiratory systems in Proteobacteria

Pascale Infossi,Chantal Iobbi-Nivol,Amine Ali Chaouche,Marie-Thérèse Giudici-Orticoni,Olivier N Lemaire,Vincent Méjean,Silke Leimkühler,Leon Espinosa

doi:10.1038/s41598-018-31851-2

Abstract

In this report, we investigate small proteins involved in bacterial alternative respiratory systems that improve the enzymatic efficiency through better anchorage and multimerization of membrane components. Using the small protein TorE of the respiratory TMAO reductase system as a model, we discovered that TorE is part of a subfamily of small proteins that are present in proteobacteria in which they play a similar role for bacterial respiratory systems. We reveal by microscopy that, in Shewanella oneidensis MR1, alternative respiratory systems are evenly distributed in the membrane contrary to what has been described for Escherichia coli. Thus, the better efficiency of the respiratory systems observed in the presence of the small proteins is not due to a specific localization in the membrane, but rather to the formation of membranous complexes formed by TorE homologs with their c-type cytochrome partner protein. By an in vivo approach combining Clear Native electrophoresis and fluorescent translational fusions, we determined the 4:4 stoichiometry of the complexes. In addition, mild solubilization of the cytochrome indicates that the presence of the small protein reinforces its anchoring to the membrane. Therefore, assembly of the complex induced by this small protein improves the efficiency of the respiratory system.

Highlights

A few years ago, Storz et al.[1] claimed that “small proteins can no longer be ignored”
It is noteworthy that TorE orthologs are found in various genders of β and γ classes of Proteobacteria, whereas in α and ε-Proteobacteria the systems are present in a few genders and do not possess any TorE homolog
Compared to the Tor system, the Nap system presents a higher distribution in all division of proteobacteria except the epsilon and delta proteobacteria, with the latter not synthetizing either of these two systems

Summary

Introduction

A few years ago, Storz et al.[1] claimed that “small proteins can no longer be ignored”. Small proteins are involved in the sporulation process like SpoVM of B. subtilis[2], affect signal transduction like MgrB and Sda[3,4], act like chaperones as described for MntS5 or FbpB6 and regulate inner membrane transporter like MgtS7 as well as the activity of membranous enzymatic complexes like CydX8–12. TMAO reduction is mainly due to the Tor system, which production is strictly controlled by the presence of TMAO in the bacterium environment[14,15] It is made up by the periplasmic TMAO reductase TorA and the pentahemic membrane-anchored c-type cytochrome TorC15,21. TorE and its homolog NapE from the periplasmic nitrate reductase respiratory system are part of a family of small proteins that are required to stabilize membrane anchored respiratory systems of certain proteobacteria

Methods

Results

Conclusion