Novel secreted STPKLRR from Vibrio splendidus AJ01 promotes pathogen internalization via mediating tropomodulin phosphorylation dependent cytoskeleton rearrangement

Abstract

We previously demonstrated that the flagellin of intracellular Vibrio splendidus AJ01 could be specifically identified by tropomodulin (Tmod) and further mediate p53-dependent coelomocyte apoptosis in the sea cucumber Apostichopus japonicus. In higher animals, Tmod serves as a regulator in stabilizing the actin cytoskeleton. However, the mechanism on how AJ01 breaks the AjTmod-stabilized cytoskeleton for internalization remains unclear. Here, we identified a novel AJ01 Type III secretion system (T3SS) effector of leucine-rich repeat-containing serine/threonine-protein kinase (STPKLRR) with five LRR domains and a serine/threonine kinase (STYKc) domain, which could specifically interact with tropomodulin domain of AjTmod. Furthermore, we found that STPKLRR directly phosphorylated AjTmod at serine 52 (S52) to reduce the binding stability between AjTmod and actin. After AjTmod dissociated from actin, the F-actin/G-actin ratio decreased to induce cytoskeletal rearrangement, which in turn promoted the internalization of AJ01. The STPKLRR knocked out strain could not phosphorylated AjTmod and displayed lower internalization capacity and pathogenic effect compared to AJ01. Overall, we demonstrated for the first time that the T3SS effector STPKLRR with kinase activity was a novel virulence factor in Vibrio and mediated self-internalization by targeting host AjTmod phosphorylation dependent cytoskeleton rearrangement, which provided a candidate target to control AJ01 infection in practice. Vibrio splendidus AJ01 is the major pathogen for skin ulcer syndrome (SUS) in Apostichopus japonicus, nevertheless, its pathogenic mechanism remains unknown. Eukaryotic-like factors play a crucial role in bacterial virulence by targeting hosts. Despite their significance, the eukaryotic factors in Vibrio splendidus have not been studied, and the mechanism by which they interact with the host remains unclear. In this study, we found an eukaryotic-like factor STPKLRR of V. splendidus for the first time. Furthermore, STPKLRR, selected by T3SS, can phosphorylate Tmod at S52, dissociate the Tmod/actin complex, cause cytoskeleton rearrangement and promote the internalization of V. splendidus. Our findings provide insight into the mechanisms underlying the internalization of V. splendidus and advance our knowledge of the general biology of pathogen-host interactions.