Bacterial expression of human small GTPases induces stochastic predatory responses and motility interference in Physarum polycephalum

We investigated the behavioral responses of the slime mold Physarum polycephalum to Escherichia coli expressing human small GTPases to elucidate how internalized proteins affect its actomyosin-based motility machinery. We found that prey bacteria expressing hKRAS WT or hRhoA triggered a deterministic “conflict” behavior, in which P. polycephalum failed to complete ingestion despite physical contact. Notably, the oncogenic hKRAS G12C mutation induced a transition to stochastic switching among predation, conflict, and sampling-mediated rejection, whereas hRac1 expression showed a tendency toward diversified behavioral outcomes. To explain this, we propose a working model centered on molecular jamming or local perturbation of intracellular oscillatory dynamics, in which non-prenylated G-domains act as decoys that decouple the contractile machinery from leading-edge actin dynamics. This behavioral stochasticity likely reflects physical noise arising from the specific conformational states and functional heterogeneity of the expressed proteins. Our results suggest that P. polycephalum functions as a sensitive biological sensor of the biophysical properties of prey proteins, providing a unique platform for exploring the co-evolutionary arms race and molecular survival strategies between bacteria and amoeboid predators.