Dugesia japonica planarian flatworms are naturally exposed to various microbes but typically survive this challenge. We show that planarians eliminate bacteria pathogenic to Homo sapiens, Caenorhabditis elegans, and/or Drosophila melanogaster and thus represent a model to identify innate resistance mechanisms. Whole-transcriptome analysis coupled with RNAi screening of worms infected with Staphylococcus aureus or Legionella pneumophila identified 18 resistance genes with nine human orthologs, of which we examined the function of MORN2. Human MORN2 facilitates phagocytosis-mediated restriction of Mycobacterium tuberculosis, L. pneumophila, and S. aureus in macrophages. MORN2 promotes the recruitment of LC3, an autophagy protein also involved in phagocytosis, to M. tuberculosis-containing phagosomes and subsequent maturation to degradative phagolysosomes. MORN2-driven trafficking of M. tuberculosis to single-membrane, LC3-positive compartments requires autophagy-related proteins Atg5 and Beclin-1, but not Ulk-1 and Atg13, highlighting the importance of MORN2 in LC3-associated phagocytosis. These findings underscore the value of studying planarian defenses to identify immune factors.

Transendothelial cell macroaperture (TEM) tunnels control endothelium barrier function and are triggered by several toxins from pathogenic bacteria that provoke vascular leakage. Cellular dewetting theory predicted that a line tension of uncharacterized origin works at TEM boundaries to limit their widening. Here, by conducting high-resolution microscopy approaches we unveil the presence of an actomyosin cable encircling TEMs. We develop a theoretical cellular dewetting framework to interpret TEM physical parameters that are quantitatively determined by laser ablation experiments. This establishes the critical role of ezrin and non-muscle myosin II (NMII) in the progressive implementation of line tension. Mechanistically, fluorescence-recovery-after-photobleaching experiments point for the upstream role of ezrin in stabilizing actin filaments at the edges of TEMs, thereby favouring their crosslinking by NMIIa. Collectively, our findings ascribe to ezrin and NMIIa a critical function of enhancing line tension at the cell boundary surrounding the TEMs by promoting the formation of an actomyosin ring.

The E3 ubiquitin ligase HACE1 is a potent tumor suppressor that controls cell proliferation and ubiquitylates the small GTPase Rac1 to target it to proteasomal degradation. Whether and how the activity of HACE1 is regulated by the N-terminal ankyrin (ANK) and the middle (MID) domains is ill defined. Here, we identified in the version 64 of the Catalogue of Somatic Mutations in Cancer (COSMIC) 13 missense mutations of hace1 located outside the HECT domain, and found that all lead to defective control of cell proliferation. In addition, several mutations located in the ankyrin domain displayed a dramatic reduction in Rac1 ubiquitylation associated with a decrease of colony formation in soft agar. 3D structure modelling of the 7 ankyrin-repeats coupled to functional analysis identified a surface epitope centered on one of the mutated residue, Gly-175, which is critical for controlling Rac1 binding and ubiquitylation. We also identified a role for the MID domain in conferring the specificity of association of HACE1 to the active form of Rac1. Our study of the functional interplay between HACE1 and Rac1 in cancer thus sheds a new light on the molecular mechanism of Rac1 ubiquitylation by HACE1 and the impact of its cancer-associated mutations in cell proliferation.