Intrinsic disorder in pathogen effectors: protein flexibility as an evolutionary hallmark in a molecular arms race

Abstract

Effector proteins represent a refined mechanism of bacterial pathogens to overcome plants' innate immune systems. These modular proteins often manipulate host physiology by directly interfering with immune signaling of plant cells. Even if host cells have developed efficient strategies to perceive the presence of pathogenic microbes and to recognize intracellular effector activity, it remains an open question why only few effectors are recognized directly by plant resistance proteins. Based on in-silico genome-wide surveys and a reevaluation of published structural data, we estimated that bacterial effectors of phytopathogens are highly enriched in long-disordered regions (>50 residues). These structurally flexible segments have no secondary structure under physiological conditions but can fold in a stimulus-dependent manner (e.g., during protein-protein interactions). The high abundance of intrinsic disorder in effectors strongly suggests positive evolutionary selection of this structural feature and highlights the dynamic nature of these proteins. We postulate that such structural flexibility may be essential for (1) effector translocation, (2) evasion of the innate immune system, and (3) host function mimicry. The study of these dynamical regions will greatly complement current structural approaches to understand the molecular mechanisms of these proteins and may help in the prediction of new effectors.

Figure 1.

Intrinsic Disorder in Effector Proteins. Bacterial effector proteins are secreted via TTSS into the host cell cytoplasm, where they manipulate host cell immune signaling and physiology. Structural flexibility is required for efficient secretion via TTSS, as proteins can only be secreted in an unfolded state. Posttranslational modifications (PTM) can be added to residues within ID segments to determine subcellular localization of the protein inside the host cell. This structural signature may also contribute to effector virulence via mimicking or interactions with signaling proteins and evasion of immune recognition by Resistance (R) proteins. Top left box: The structural model of HopPmaL_281-385 (PDB_ID: 2LF3; inset top left corner) illustrates the structural flexibility of ID regions. The unfolded region is depicted in orange.