Bartonella quintana type IV secretion effector BepE-induced selective autophagy by conjugation with K63 polyubiquitin chain.

Bartonella effector proteins (named Beps) are substrates of VirB type IV secretion system for translocation into host cells evolved in Bartonella spp. Among these, BepE has been shown to protect cells from fragmentation effects triggered by other Beps and to promote in vivo dissemination of bacteria from the dermal site of inoculation to the bloodstream. Bacterial pathogens secreted effectors to modulate the interplay with host autophagy, either to combat autophagy to escape its bactericidal effect or to exploit autophagy to benefit intracellular replication. Here, we reported a distinct phenotype that selective autophagy in host cells is activated as a countermeasure, to attack BepE via conjugation with K63 polyubiquitin chain on BepE. We found that ectopic expression of Bartonella quintana BepE specifically induced punctate structures that colocalised with an autophagy marker (LC3-II) in host cells, in addition to filopodia and membrane ruffle formation. Two tandemly arranged Bartonella Intracellular Delivery (BID) domains in the BepE C-terminus, where ubiquitination of sister pairs of lysine residues was confirmed, were essential to activate host cell autophagy. Multiple polyubiquitin chain linkages of K27, K29, K33, and K63 were found to be conjugated at sites of K222 and K365 on BepE, of which K63 polyubiquitination on BepE K365 determined the selective autophagy (p62/SQSTM1 positive autophagy) independent of the PI3K pathway. Colocalisation of BepE with LAMP1 confirmed the maturation of BepE-induced autophagosomes in which BepE were targeted for degradation. Moreover, host cells employed selective autophagy to counter-attack BepE to rescue cells from BepE-induced endocytosis deficiency.