The EspF effector, a bacterial pathogen's Swiss army knife

Abstract

Central to the pathogenesis of many bacterial pathogens is the ability to deliver effector proteins directly into the cells of their eukaryotic host. EspF is one of many effector proteins exclusive to the attaching and effacing pathogen family that includes enteropathogenic (EPEC) and enterohemorrhagic (EHEC) Escherichia coli. Work in recent years has revealed EspF to be one of the most multifunctional effector proteins known, with defined roles in several host cellular processes, including disruption of the epithelial barrier, antiphagocytosis, microvillus effacement, host membrane remodelling, modulation of the cytoskeleton, targeting and disruption of the nucleolus, intermediate filament disruption, cell invasion, mitochondrial dysfunction, apoptosis, and inhibition of several important epithelial transporters. Surprisingly, despite this high number of functions, EspF is a relatively small effector protein, and recent work has begun to decipher the molecular events that underlie its multifunctionality. This review focuses on the activities of EspF within the host cell and discusses recent findings and molecular insights relating to the virulence functions of this fascinating bacterial effector.

FIG. 1.

Modular architecture of EspF. (A) The different sizes of the four main EspF variants are determined by their number of proline-rich repeats (PRR), which are almost identical in size and sequence similarity. The EPEC and EHEC strains relate to E2348/69 and EDL933, respectively. (B) Phylogenetic tree of the N-terminal EspF region (residues 1 to 73) produced by using Clustal W software. Percent values indicate sequence identity relative to the EPEC variant. (C) Modular structure of the N-terminal region (residues 1 to 73) comprising the secretion signal, mitochondrial targeting signal (MTS), and nucleolar targeting domain. (D) Typical EspF PRR module taken from EPEC EspF, showing the host protein binding motifs; SNX9, sorting nexin 9.

FIG. 2.

The multiple functions of EspF. (A) Table listing all known functions of EspF in A/E pathogens as described in the text. The far-left column indicates whether evidence for the EspF function is direct (host cell expression or biochemical studies) or related to infection with EspF-deficient bacterial mutants. (B) Diagram of intestinal cells illustrating the varied functions and subcellular behavior of EspF. Parentheses indicate the host protein interaction predicted or proven to play a role in the indicated function.

FIG. 3.

Subcellular locations of EspF. (A) Table listing the predicted and proven subcellular locations for EspF's multiple functions, based on the assumption that EspF only targets the three locations given, although other sites are likely. Mito., mitochondrial; Nuc., nucleolar. (B) Colocalization of EspF with the mitochondrial marker DsRed in EPEC-infected HeLa cells. (C) EspF present in the mitochondria and nucleolus in late-stage-infected HeLa cells. (D) Nucleolar accumulation of the EspF variant (L16E) that cannot target the mitochondria. (E) Western blot (WB) showing EspF in both the mitochondrial (Mito.) and cytoplasmic (Cyto.) fractions of Caco-2 cells at all stages of infection. (F) Processing of EspF within the mitochondrial fraction of infected intestinal cells is dependent on an intact mitochondrial membrane potential (a) and is prevented with the mitochondrial inhibitor valinomycin (b).