Cytochalasin D (CyD) is a drug that binds to the plus end of actin filaments and prevents actin polymerization. Having identified mutations in actin that confer resistance to CyD, a researcher sets out to study the role of the actin cytoskeleton in the invasion of mammalian host cells by two intracellular parasites: the protozoan Toxoplasma gondii and the bacterium Salmonella enterica. She infects wild-type or CyD-resistant host cells with either wild-type T. gondii, CyD-resistant T. gondii, or wild-type S. enterica, each in the presence (+) or absence (-) of CyD, and measures parasite internalization and intracellular proliferation as a result. The findings are summarized in the table below, in which High or Low levels of proliferation are indicated. According to these results, which column (A to C) do you think corresponds to infection with wild-type T. gondii? Which column corresponds to CyD-resistant T. gondii? Which row (D or E) corresponds to the wild-type host? Your answer would be a three-letter string composed of letters A to E only, e.g. ABE.

Question

Quizplus · Accepted Answer

In contrast to intracellular bacterial pathogens such as S. enterica, intracellular eukaryotic parasites such as T. gondii actively invade host cells. The energy of active invasion seems to come from actin polymerization in the parasite rather than the host. It follows that interfering with actin dynamics in the parasite (but not the host) using CyD would inhibit internalization. CyD-resistant T. gondii (column C) can therefore invade both host-cell types whether or not CyD is present. Wild-type T. gondii (column B), in contrast, can efficiently infect the hosts only in the absence of the drug. Finally, wild-type S. enterica (column A) proliferates inside the hosts under all conditions except in the wild-type host (row D) in the presence of CyD.

Cytochalasin D (CyD) Is a Drug That Binds to the Plus