Abstract

As the C. elegans oocyte begins to develop, it must first fill the region just under its plasma membrane with a contractile and dynamic meshwork of actomyosin filaments. How is this first actomyosin cortex put together from constituent molecules? I will describe the discovery that in C. elegans oocytes, actomyosin cortical assembly relies on the emergence of thousands of short-lived protein condensates rich in actin filaments, and filament nucleators. We extract empirical growth laws governing the composition dependent chemical dynamics of these condensates. We show that  in contrast to condensate growth via diffusion – here, condensate growth is chemically driven. The associated chemical reactions obey mass action kinetics despite governing both composition and size. Remarkably, this coupling of mass action kinetics and assembly kinetics highlights several novel principles of intracellular physical chemistry likely to be applicable across cell biology.