Collision of Expanding Actin Caps with Actomyosin Borders for Cortical Bending and Mitotic Rounding in a Syncytium

Publication date: Available online 24 May 2018
Source:Developmental Cell
Author(s): Yixie Zhang, Jessica C. Yu, Tao Jiang, Rodrigo Fernandez-Gonzalez, Tony J.C. Harris
The early Drosophila embryo is a large syncytial cell that compartmentalizes mitotic spindles with furrows. Before furrow ingression, an Arp2/3 actin cap forms above each nucleus and is encircled by actomyosin. We investigated how these networks transform a flat cortex into a honeycomb-like compartmental array. The growing caps circularize and ingress upon meeting their actomyosin borders, which become the furrow base. Genetic perturbations indicate that the caps physically displace their borders and, reciprocally, that the borders resist and circularize their caps. These interactions create an actomyosin cortex arrayed with circular caps. The Rac-GEF Sponge, Rac-GTP, Arp3, and actin coat the caps as a growing material that can drive cortical bending for initial furrow ingression. Additionally, laser ablations indicate that actomyosin contraction squeezes the cytoplasm, producing counterforces that swell the caps. Thus, Arp2/3 caps form clearances of the actomyosin cortex and control buckling and swelling of these clearances for metaphase compartmentalization.

Graphical abstract

Teaser

For mitosis in the syncytial Drosophila embryo, each nucleus organizes a surface Arp2/3 actin cap encircled by an actomyosin border. Zhang et al. show that the assembling caps and borders engage each other physically to pattern the syncytial cortex in 2D and to generate compartments for mitotic spindles in 3D.


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