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Distinct roles of Chlamydia trachomatis effectors TarP and TmeA in the regulation of formin and Arp2/3 during entry.

The obligate intracellular pathogen Chlamydia trachomatis manipulates the host actin cytoskeleton to assemble actin-rich structures that drive pathogen entry. The recent discovery of TmeA, which like TarP is an invasion-associated type III effector implicated in actin remodeling, raised questions regarding the nature of their functional interaction. Quantitative live-cell imaging of actin remodeling at invasion sites revealed differences in recruitment and turnover kinetics associated with TarP and TmeA pathways, with the former accounting for most of the robust actin dynamics at invasion sites. TarP-mediated recruitment of the actin nucleators formin and the Arp2/3 complex were crucial for rapid actin kinetics, generating a collaborative positive feedback loop that enhanced their respective actin-nucleating activities within invasion sites. In contrast, Fmn1 is neither recruited to invasion sites nor collaborates with Arp2/3 within the context of TmeA-associated actin recruitment. While the TarP-Fmn1-Arp2/3 signaling axis is responsible for the majority of actin dynamics, its inhibition had similar effects as deletion of TmeA on invasion efficiency, consistent with the proposed model that TarP and TmeA acting on different stages of the same invasion pathway.