Arp2/3-dependent pseudocleavage [correction of psuedocleavage] furrow assembly in syncytial Drosophila embryos.
BACKGROUND
In syncytial blastoderm Drosophila embryos, actin caps assemble during telophase. As the cell cycle progresses through interphase, these small caps expand and fuse to form pseudocleavage furrows that are structurally related to the cleavage furrows that assemble during somatic cell division. The molecular mechanism driving cell cycle coordinated actin reorganization from the caps to the furrows is not understood.


RESULTS
We show that Drosophila embryos contain a typical Arp2/3 complex and that components of this complex localize to the margins of the expanding caps, to mature pseudocleavage furrows, and to somatic cell cleavage furrows during the postcellularization embryonic divisions. A mutation that disrupts the arpc1 subunit of Arp2/3 leads to spindle fusions that are characteristic of pseudocleavage furrow disruption. By contrast, this mutation does not significantly affect nuclear positioning during interphase, which is dependent on actin cap function. In vivo analysis of actin reorganization demonstrates that the arpc1 mutation does not prevent assembly of small actin caps but blocks cap expansion and furrow assembly as the cell cycle progresses through interphase. The scrambled gene is also required for cap expansion and furrow assembly, and Scrambled is required for Arp2/3 localization to the cap margins.


CONCLUSIONS
The Drosophila Arp2/3 complex and Scrambled protein are required for actin cap expansion and pseudocleavage furrow formation during the syncytial blastoderm divisions. We propose that Scrambled-dependent localization of Arp2/3 to the margins of the expanding caps triggers local actin polymerization that drives cap expansion and pseudocleavage furrow assembly.
Stevenson
Victoria
V
Program in Molecular Medicine, University of Massachusetts School of Medicine, Worcester, MA 01655, USA.
Hudson
Andrew
A
Cooley
Lynn
L
Theurkauf
William E
WE
D051377:Actin-Related Protein 2; D051378:Actin-Related Protein 3; D000199:Actins / Q000378:metabolism*; D000818:Animals; D001756:Blastoderm / Q000502:physiology; D002453:Cell Cycle / Q000502:physiology; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000378:metabolism*; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000196:embryology* / Q000235:genetics / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000502:physiology*; D018507:Gene Expression Regulation, Developmental; D019476:Insect Proteins / Q000235:genetics / Q000378:metabolism; D018613:Microscopy, Confocal; D009154:Mutation
D051377:Actin-Related Protein 2; D051378:Actin-Related Protein 3; D000199:Actins; D018797:Cell Cycle Proteins; D003598:Cytoskeletal Proteins; D029721:Drosophila Proteins; D019476:Insect Proteins; C421006:sced protein, Drosophila