Abstract
Gene clusters encoding various type III secretion system (T3SS) injectisomes, frequently code downstream of the conserved atpase gene for small hydrophilic proteins whose amino acid sequences display a propensity for intrinsic disorder and coiled-coil formation. These properties were confirmed experimentally for a member of this class, the HrpO protein from the T3SS of Pseudomonas syringae pv phaseolicola: HrpO exhibits high alpha-helical content with coiled-coil characteristics, strikingly low melting temperature, structural properties that are typical for disordered proteins, and a pronounced self-association propensity, most likely via coiled-coil interactions, resulting in heterogeneous populations of quaternary complexes. HrpO interacts in vivo with HrpE, a T3SS protein for which coiled-coil formation is also strongly predicted. Evidence from HrpO analogues from all T3SS families and the flagellum suggests that the extreme flexibility and propensity for coiled-coil interactions of this diverse class of small, intrinsically disordered proteins, whose structures may alter as they bind to their cognate folded protein targets, might be important elements in the establishment of protein-protein interaction networks required for T3SS function.
Highlights
That translocate a diverse repertoire of proteins either to extracellular locations or directly into eukaryotic cells, in a Sec-independent manner
The extreme flexibility of intrinsically disordered proteins, whose structures may alter dramatically as they bind to their cognate folded protein target, has been suggested to represent a strategy for optimizing the search and interaction with their targets [47]
This strategy may apply to the binding of HrpO to its protein targets, as suggested by its pronounced flexibility and the observed ␣-helical ordering effects associated with the HrpO/HrpE interaction
Summary
That translocate a diverse repertoire of proteins (effectors) either to extracellular locations or directly into eukaryotic cells, in a Sec-independent manner (interkingdom protein transfer). The HrpO Sequence Exhibits Characteristics of an Intrinsically Disordered Coil-coil and Has Extensive Analogies to Proteins from Other T3SS and the Flagellum—Despite the absence of a significant homology (the residue identity is less than 18%), the HrpO and FliJ sequences share specific characteristics, which suggest similar properties: Based on their average hydrophobicity and net charge, both proteins are predicted [29] to be unfolded and highly ␣-helical with a pronounced coiled-coil propensity.
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