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*Listeria Infections
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© The Rockefeller University Press, 0021-9525/1997//155 $5.00
The Journal of Cell Biology, Volume 137, Number 1, , 1997 155-167

Article

The Isolated Comet Tail Pseudopodium of Listeria monocytogenes: A Tail of Two Actin Filament Populations, Long and Axial and Short and Random



Antonio S. Sechi*, Jürgen Wehland{ddagger}, and J. Victor Small*

* Institute of Molecular Biology, Austrian Academy of Sciences, A-5020 Salzburg, Austria; and {ddagger} GBF, Gesellschaft für Biotechnologische Forschung, Department of Cell Biology and Immunology, D-38124 Braunschweig, Germany

Listeria monocytogenes is driven through infected host cytoplasm by a comet tail of actin filaments that serves to project the bacterium out of the cell surface, in pseudopodia, to invade neighboring cells. The characteristics of pseudopodia differ according to the infected cell type. In PtK2 cells, they reach a maximum length of ~15 µm and can gyrate actively for several minutes before reentering the same or an adjacent cell. In contrast, the pseudopodia of the macrophage cell line DMBM5 can extend to >100 µm in length, with the bacteria at their tips moving at the same speed as when at the head of comet tails in bulk cytoplasm. We have now isolated the pseudopodia from PtK2 cells and macrophages and determined the organization of actin filaments within them. It is shown that they possess a major component of long actin filaments that are more or less splayed out in the region proximal to the bacterium and form a bundle along the remainder of the tail. This axial component of filaments is traversed by variable numbers of short, randomly arranged filaments whose number decays along the length of the pseudopodium. The tapering of the tail is attributed to a grading in length of the long, axial filaments.

The exit of a comet tail from bulk cytoplasm into a pseudopodium is associated with a reduction in total F-actin, as judged by phalloidin staining, the shedding of {alpha}-actinin, and the accumulation of ezrin. We propose that this transition reflects the loss of a major complement of short, random filaments from the comet, and that these filaments are mainly required to maintain the bundled form of the tail when its borders are not restrained by an enveloping pseudopodium membrane. A simple model is put forward to explain the origin of the axial and randomly oriented filaments in the comet tail.

The light microscope and imaging facility used for this work was purchased from funds granted by the Austrian National Bank and the Seegen Stifung of the Austrian Academy of Sciences, which are gratefully acknowledged.

1. Abbreviation used in this paper: S-1, subfragment 1.

Please address all correspondence to J. Victor Small, Institute of Molecular Biology, Austrian Academy of Sciences, Billrothstrasse 11, A-5020 Salzburg, Austria. Tel.: (43) 662-63961-11. Fax: (43) 662-63961-40. e-mail: jvsmall{at}edvz.sbg.ac.at

A.S. Sechi's present address for reprints is Tel.: (49)-531-6181-415. Fax: (49) 531-6181-444. e-mail: ase{at}gbf-braunschweig.de


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