A function of profilin in force generation during malaria parasite motility independent of actin binding

Catherine A. Moreau, Katharina A. Quadt, Henni Piirainen, Hirdesh Kumar, Saligram P. Bhargav, Léanne Strauss, Niraj H. Tolia, Rebecca C. Wade, Joachim P. Spatz, Inari Kursula, Friedrich Frischknecht

During transmission of malaria-causing parasites from mosquito to mammal, Plasmodium sporozoites migrate at high speed within the skin to access the bloodstream and infect the liver. This unusual gliding motility is based on retrograde flow of membrane proteins and highly dynamic actin filaments that provide short tracks for a myosin motor. Using laser tweezers and parasite mutants, we previously suggested that actin filaments form macromolecular complexes with plasma-membrane spanning adhesins to generate force during migration. Mutations in the actin-binding region of profilin, a near ubiquitous actin-binding protein, revealed that loss of actin binding also correlates with loss of force production and motility. Here we show that different mutations in profilin, not affecting actin binding in vitro, still generate lower force during Plasmodium sporozoite migration. Lower force generation inversely correlates with increased retrograde flow suggesting that, like in mammalian cells, the slow-down of flow to generate force is the key underlying principle governing Plasmodium gliding motility.

DOI

Low-force transitions in single titin molecules reflect a memory of contractile history

Zsolt Mártonfalvi, Pasquale Bianco, Marco Linari, Marco Caremani, Attila Nagy, Vincenzo Lombardi and Miklós Kellermayer

Titin, a giant elastomeric muscle protein has been implicated to function as a sensor of sarcomeric stress and strain but with unresolved mechanisms. To gain insight into titin's mechanosensory function here we manipulated single molecules with high-resolution optical tweezers. Discrete, stepwise transitions, with rates faster than canonical Ig-domain unfolding occurred during stretch at forces as low as 5 pN. Multiple mechanisms and molecular regions (PEVK, proximal tandem-Ig, N2A) are likely to be involved. The pattern of transitions is sensitive to the history of contractile events. Monte-Carlo simulations recovered our experimental results and predicted that structural transitions may begin prior to the complete extension of the PEVK domain. High-resolution AFM of titin extended with meniscus forces supported this prediction. Addition of glutamate-rich PEVK-domain fragments competitively inhibited the viscoelastic response in both single titin molecules and muscle fibers, indicating that intra-PEVK-domain interactions contribute significantly to sarcomere mechanics. Thus, under non-equilibrium conditions across the physiological force range, titin extends via a complex pattern of history-dependent discrete conformational transitions which, by dynamically exposing ligand-binding sites, may set the stage for the biochemical sensing of the sarcomeric mechanical status.

DOI

Filopodium retraction is controlled by adhesion to its tip

Stephane Romero, Alessia Quatela, Thomas Bornschlögl, Stéphanie Guadagnini, Patricia Bassereau and Guy Tran Van Nhieu

Filopodia are thin cell extensions sensing the environment. They play an essential role during cell migration, cell-cell or cell-matrix adhesion, by initiating contacts and conveying signals to the cell cortex. Pathogenic microorganisms can hijack filopodia to invade cells by inducing their retraction towards the cell body. Because their dynamics depend on a discrete number of actin filaments, filopodia provide a model of choice to study elementary events linked to adhesion and downstream signaling. However, the determinants controlling filopodial sensing are not well characterized. Here, we have used beads functionalized with different ligands that triggered filopodial retraction when contacting filopodia of epithelial cells. With optical tweezers (OTs), we were able to measure forces stalling the retraction of a single filopodium. We found that the filopodial stall force depends on the coating of the bead. Stall forces reached 8 pN for beads coated with the β1- integrin ligand Yersinia Invasin, while retraction was stopped with a higher force of 15 pN when beads were functionalized with carboxyl groups. In all cases, stall forces increased in correlation with the density of ligands contacting filopodial tips and were independent of the optical trap stiffness. Unexpectedly, a discrete and small number of Shigella type three secretion systems induced stall forces of 10 pN. These results suggest that the number of receptor-ligand interactions at the filopodial tip determines the maximal retraction force exerted by filopodia but a discrete number of clustered receptors is sufficient to induce high retraction stall forces.

DOI