Identification of chaperone-independent outer membrane proteins and MtrA-assisted MtrB folding in Shewanella oneidensis

Extracellular electron transfer is a respiratory process conducted by a number of microorganisms in order to access insoluble or membrane impermeable electron acceptors or donors. The process has wide implication for the biogeochemistry of our planet and offers many opportunities for biotechnological applications. Outer membrane spanning electron transfer is conducted by the model organism Shewanella oneidensis MR-1 by a stable trimeric protein complex. The electron conduit consists of two c-type cytochromes on either site of the outer membrane and a {beta}-barrel protein in the middle that seems to facilitate interaction of the two heme containing proteins. This study reveals that the periplasmic c-type cytochrome MtrA is not only part of the electron conduit, but also assists in the periplasmic transport of the unfolded outer membrane protein MtrB, a function that was so far believed to be conducted for all outer membrane {beta}-barrel proteins by one of the canonical chaperones SurA, Skp or DegP. However, three more {beta}-barrel proteins were identified that are independent of the canonical chaperones as well but still rely on the BAM complex in the outer membrane, suggesting that many more solutions for periplasmic transfer of {beta}-barrel protein towards the outer membrane exist in Gram-negative microorganisms.