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J. Cell Biol.,
Volume 143, Number 1, October 5, 1998 121-133
Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104
In developing Drosophila bristles two species of cross-linker, the forked proteins and fascin, connect adjacent actin filaments into bundles. Bundles
form in three phases: (a) tiny bundles appear; (b) these
bundles aggregate into larger bundles; and (c) the filaments become maximally cross-linked by fascin. In mutants that completely lack forked, aggregation of the
bundles does not occur so that the mature bundles consist of <50 filaments versus ~700 for wild type. If the
forked concentration is genetically reduced to half the
wild type, aggregation of the tiny bundles occurs but
the filaments are poorly ordered albeit with small
patches of fascin cross-linked filaments. In mutants
containing an excess of forked, all the bundles tend to
aggregate and the filaments are maximally crossbridged
by fascin. Alternatively, if fascin is absent, phases 1 and
2 occur normally but the resultant bundles are twisted
and the filaments within them are poorly ordered. By
extracting fully elongated bristles with potassium iodide which removes fascin but leaves forked, the bundles change from being straight to twisted and the filaments within them become poorly ordered. From these observations we conclude that (a) forked is used early
in development to aggregate the tiny bundles into
larger bundles; and (b) forked facilitates fascin entry
into the bundles to maximally cross-link the actin filaments into straight, compact, rigid bundles. Thus, forked aligns the filaments and then directs fascin binding so that inappropriate cross-linking does not occur.
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