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Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104
At a late stage in Drosophila oogenesis,
nurse cells rapidly expel their cytoplasm into the oocyte
via intracellular bridges by a process called nurse cell
dumping. Before dumping, numerous cables composed
of actin filaments appear in the cytoplasm and extend
inward from the plasma membrane toward the nucleus.
This actin cage prevents the nucleus, which becomes
highly lobed, from physically blocking the intracellular
bridges during dumping. Each cable is composed of a
linear series of modules composed of ~25 cross-linked
actin filaments. Adjacent modules overlap in the cable
like the units of an extension ladder. During cable formation, individual modules are nucleated from the cell
surface as microvilli, released, and then cross-linked to
an adjacent forming module. The filaments in all the
modules in a cable are unidirectionally polarized. During dumping as the volume of the cytoplasm decreases,
the nucleus to plasma membrane distance decreases, compressing the actin cables that shorten as adjacent
modules slide passively past one another just as the elements of an extension ladder slide past one another for
storage. In Drosophila, the modular construction of actin cytoskeletons seems to be a generalized strategy.
The behavior of modular actin cytoskeletons has implications for other actin-based cytoskeletal systems, e.g.,
those involved in Listeria movement, in cell spreading,
and in retrograde flow in growth cones and fibroblasts.
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