The Bacterial Cytoskeleton based on Bacterial Translation Elongation Factor EF-Tu: Novel Insights

Bacteria possess an EF-Tu-based cytoskeleton.This article presents a short review. A number of questions which are not discussed in the former publications can be asked, such as: all bacteria possess a ribosomal protein synthesis system and, hence, also EF-Tu. EF-Tu is produced in an amount that is higher than the need for a function as translation elogation factor in ribsomal protein synthesis. This article tries to answer the question regarding the surplus of EF-Tu: formation of a "cell-wide web" by self-assembly as a feafure that stabilizes cell integrity. An additional question can be asked: what is the origin of this bacterial cytoskeleton? This article contains a speculation on this topic. A third question regards the'ntteructjon of ribosomes in the process of protemsynthesis: does the EF-Tu protein move to the ribosome, or does the ribosome move to the EF-Tu intergated in a fibril of the bacterial cytoskeleton? The former publication depicts electron micrographs which show colocalizatton of botth entities. EF-Tu is an example for aprotein with two independent functions: participation in the ribosomal protein synthesis as a kanslation elongation factor, and component of a bacterial cytoskeleton. This situation can open up a discussion ofthe sequence of events and states of early cells during evolution.

Autotransporter secretion exploits the bacterial actin-homologue

Bacterial infection of humans, animals and plants relies heavily on secreted proteases that degrade host defences or activate bacterial toxins. The largest family of proteins secreted by Gram-negative pathogenic bacteria, the Autotransporters (ATs), includes key proteolytic virulence factors. There remains uncertainty about the mechanistic steps of the pathway ATs share to exit bacteria, and how it is energetically driven. This study set out to shed light on the AT secretion pathway with the ultimate aim of uncovering novel antimicrobial targets that would be unlikely to trigger the development of resistance mechanisms in bacteria. To do this, two AT virulence factors with distinct proteolytic functions, EspC (secreted from EnteropathogenicEscherichia coli) and AaaA (tethered to the extracellular surface ofPseudomonas aeruginosa) were chosen. EspC and AaaA were fluorescently labelled using two separate methods to establish the localization patterns of ATs as they are secreted from a bacterial cell. Super resolution microscopy revealed that localization of ATs occurs via a helical route along the bacterial cytoskeleton. In addition to requiring the conserved C-terminal β-barrel translocator domain of the AT, we present the first evidence that secretion is dependent on a dynamic interaction with a structure reliant upon the actin homologue MreB and the Sec translocon. These findings provide a step forward in the mechanistic understanding of the secretion of this widely distributed family of proteins that have pivotal roles in bacterial pathogenesis and conserved structural properties that could serve as novel broad-range antimicrobial targets.SignificanceSecreted bacterial proteases facilitate the infection of human, animal and plant hosts by degrading host defences or activating bacterial toxins. The autotransporter family is the largest family of proteins secreted from Gram-negative bacteria, and includes proteolytic virulence factors crucial to bacterial infection. Precisely how autotransporters migrate from the inside to the outside of the cell, and how this movement is energetically driven is a mystery. We demonstrate a spiral pathway of autotransporter secretion, presenting evidence that it involves a dynamic interaction with the actin homologue MreB that comprises the bacterial cytoskeleton. Our findings open the way to unravelling the mechanism of autotransporter secretion and offer the possibility to identify novel antimicrobial targets unlikely to trigger the development of antimicrobial resistance.