Question

4) Studying the motility of the pathogenic bacteria Listeria and Shigella, which cause food poisoning and dysentery, respectively, significantly advanced our understanding of actin polymerization. These bacteria escape immune surveillance by entering one cell’s cytoplasm and then spreading from cell to cell without exposing themselves to the outside environment. They harness actin polymerization to push themselves against the plasma membrane, generating a membrane protrusion that is engulfed by a neighboring cell, giving them direct access to that cell’s cytoplasm. The comet tail of actin filaments in the wake of a moving Listeria bacterium initially suggested that motility was actin based. Biochemical studies identified host-cell surface proteins required for polymerization of actin, which then led to the discovery of the Arp2/3 complex and the full in vitro reconstitution of bacterial motility.

            a. Early analysis in Listeria pointed to the ActA protein, an integral membrane protein expressed on the cell surface, as the sole Listeria protein required for motility. Which combination of the following observations provides evidence that ActA is necessary and sufficient for motility?

                        A. ActA binds to the Arp2/3 complex

                        B. E. coli expressing ActA can move in host-cell cytoplasm

                        C. Listeria lacking the ActA gene fail to move in host-cell cytoplasm

                        i. A and B

                        ii. A and C

                        iii. B and C

                        iv. A, B, and C

            b. The mechanism of bacterial motility was initially mysterious and controversial: Was actin polymerization sufficient or did motility require actin-based motor proteins, as well? Which one of the following observations suggests that actin polymerization by itself generates the crucial force that drives motility.

                        i. Bacteria that lack ActA do not show motility.

                        ii. Drugs that depolymerize actin block motility

                        iii. Inhibiting myosins does not inhibit motility.

                        iv. The Arp2/3 complex is required for motility.

            c. Efficient movement of both Shigella and Listeria in a reconstituted system requires the protein cofilin. Which one of the following actions describes the correct role for cofilin in bacterial motility?

                        i. Blocks hydrolysis of ATP by actin to preserve the ATP cap.

                        ii. Bundles actin filaments to maintain a tight comet tail of actin.

                        iii. Depolymerizes actin filaments to allow subunits to recycle.

                        iv. Promotes actin polymerization to propel bacteria forward.

Answer 1

a. Both B and C

E. coli expressing ActA can move in host-cell cytoplasm.

Listeria lacking the ActA gene fail to move in host-cell cytoplasm.

b. Inhibiting myosins does not inhibit motility.

c. Depolymerizes actin filaments to allow subunits to recycle