Integrins on eggs: the βC subunit is essential for formation of the cortical actin cytoskeleton in sea urchin eggs

Eggs of several metazoans have been demonstrated to express integrins; however, their function is unclear. Previous studies have shown that the βC integrin subunit is expressed on unfertilized sea urchin eggs and proteolytically removed at fertilization. Here we report that the βC subunit is reexpressed on the egg surface immediately after fertilization. Using morpholino antisense oligonucleotides to block translation, we show that without βC expression, eggs undergo cleavage resulting in loosely adherent cells that fail to develop beyond a blastula. Without βC containing integrins, the cortical actin network of the egg does not form, yet contractile rings appear. Coinjection of RNA encoding the βC or chicken β1 subunit, but lacking the morpholino target sequence, rescues the cortical actin network and normal embryos result. Coinjection of RNA encoding the βC subunit lacking the cytoplasmic domain fails to rescue. These studies demonstrate that the cortical actin cytoskeleton is anchored by βC integrins and contractile ring actin is not. We suggest that one important function of egg integrins is to organize the actin cortex.