A circular zone of attachment to the extracellular matrix provides directionality to the motility ofToxoplasma gondiiin 3D

Abstract

Toxoplasma gondii is a protozoan parasite that infects 30-40% of the world's population. Infections are typically subclinical but can be severe and, in some cases, life threatening. Central to the virulence of T. gondii is an unusual form of substrate-dependent motility that enables the parasite to invade cells of its host and to disseminate throughout the body. A hetero-oligomeric complex of proteins that functions in motility has been characterized, but how these proteins work together to drive forward motion of the parasite remains controversial. A key piece of information needed to understand the underlying mechanism(s) is the directionality of the forces that a moving parasite exerts on the external environment. The linear motor model of motility, which has dominated the field for the past two decades, predicts continuous anterior-to-posterior force generation along the length of the parasite. We show here using three-dimensional traction force mapping that the predominant forces around a moving parasite are instead periodic and directed in towards the parasite at a fixed circular location within the extracellular matrix. These highly localized forces, which are generated by the parasite pulling on the matrix, create a visible constriction in the parasite's plasma membrane. We propose that this circular ring of inward-directed force corresponds to a circumferential attachment zone between the parasite and the matrix, through which the parasite passes as it moves forward. In parasites lacking the major surface adhesin, TgMIC2, neither the inward-directed forces nor the constriction of the parasite membrane are observed. TgMIC2-deficient parasites remain capable of moving at speeds similar to wildtype but the mutant parasites move in a less directed fashion, suggesting that TgMIC2-mediated circumferential adhesion of the parasite to the extracellular matrix serves as a guidance system to maximize forward displacement during parasite motility.