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