Mitochondrial-bacterial hybrids of BamA/Tob55 suggest variable requirements for the membrane integration of β-barrel proteins.

Anna-Katharina Pfitzner,Nadja Steblau,Doron Rapaport,Philipp Oberhettinger,Monika Schütz,Ingo B Autenrieth,Thomas Ulrich

doi:10.1038/srep39053

Anna-Katharina Pfitzner, Nadja Steblau + Show 5 more

Open Access

https://doi.org/10.1038/srep39053

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Journal: Scientific Reports	Publication Date: Dec 16, 2016
Citations: 7	License type: CC BY 4.0

Affiliation: University of Tübingen

Abstract

β-Barrel proteins are found in the outer membrane (OM) of Gram-negative bacteria, chloroplasts and mitochondria. The assembly of these proteins into the corresponding OM is facilitated by a dedicated protein complex that contains a central conserved β-barrel protein termed BamA in bacteria and Tob55/Sam50 in mitochondria. BamA and Tob55 consist of a membrane-integral C-terminal domain that forms a β-barrel pore and a soluble N-terminal portion comprised of one (in Tob55) or five (in BamA) polypeptide transport-associated (POTRA) domains. Currently the functional significance of this difference and whether the homology between BamA and Tob55 can allow them to replace each other are unclear. To address these issues we constructed hybrid Tob55/BamA proteins with differently configured N-terminal POTRA domains. We observed that constructs harboring a heterologous C-terminal domain could not functionally replace the bacterial BamA or the mitochondrial Tob55 demonstrating species-specific requirements. Interestingly, the various hybrid proteins in combination with the bacterial chaperones Skp or SurA supported to a variable extent the assembly of bacterial β-barrel proteins into the mitochondrial OM. Collectively, our findings suggest that the membrane assembly of various β-barrel proteins depends to a different extent on POTRA domains and periplasmic chaperones.

Highlights

POTRA domains 1-4 of Yersinia enterocolitica BamA were fused to fulllength Tob[55] (Fig. 1a, variant B), the second one contains POTRA domains 1-5 of BamA upstream of the membrane-embedded domain of Tob[55], in the third hybrid the single POTRA domain of Tob[55] was replaced by POTRA domain 5 of BamA
Plasmids encoding Tob[55], BamA or their hybrid proteins were transformed into yeast wild type cells and the effect of these constructs on the growth of the cells was monitored
We previously demonstrated that bacterial outer membrane proteins (OMPs) can be integrated into the mitochondrial OM upon their expression in yeast cells and that this assembly depends on the topogenesis of outer membrane β-barrel proteins (TOB) complex[15]

Summary

Introduction

It can be speculated that in the process of organelle evolution the mitochondrial BamA homolog, Tob[55], evolved in a way that it retained the most C-terminal POTRA domain as a minimal motif for β-barrel assembly. Very little is known about the exact mechanism by which the individual POTRA domains assist in the assembly of β-barrel proteins It seems that the POTRA domains facilitate the transfer of the β-barrel substrates from chaperones to the translocase at least for mitochondria, a role in the release of the precursor from the TOB complex was suggested[24,27,28]. A recent study suggested that the POTRA and the barrel domains of bacterial BamA have to match each other to ensure optimal function of the protein[29]

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