Mass Balance Imaging: A Phase Portrait Analysis for Characterizing Growth Kinetics of Biomolecular Condensates.

Biomolecular condensation has emerged as a key organizing principle governing the formation of membraneless cellular assemblies. Revealing the mechanism of formation of biomolecular condensates requires the quantitative examination of their growth kinetics. Here, we introduce mass balance imaging (MBI) as a general method to study compositional growth dynamics based on fluorescent images of multicomponent clusters. MBI allows the visualization and measurement of composition-dependent growth rates of biomolecular condensates and other assemblies. We provide a computational pipeline and demonstrate the applicability of our method by investigating cortical assemblies containing N-WASP (WSP-1) and F-actin that appear during oocyte cortex activation in C. elegans. In general, the method can be broadly implemented to identify interactions that underlie growth kinetics of multicomponent assemblies in vivo and in vitro.

A condensate dynamic instability orchestrates actomyosin cortex activation.

A key event at the onset of development is the activation of a contractile actomyosin cortex during the oocyte-to-embryo transition1-3. Here we report on the discovery that, in Caenorhabditis elegans oocytes, actomyosin cortex activation is supported by the emergence of thousands of short-lived protein condensates rich in F-actin, N-WASP and the ARP2/3 complex4-8 that form an active micro-emulsion. A phase portrait analysis of the dynamics of individual cortical condensates reveals that condensates initially grow and then transition to disassembly before dissolving completely. We find that, in contrast to condensate growth through diffusion9, the growth dynamics of cortical condensates are chemically driven. Notably, the associated chemical reactions obey mass action kinetics that govern both composition and size. We suggest that the resultant condensate dynamic instability10 suppresses coarsening of the active micro-emulsion11, ensures reaction kinetics that are independent of condensate size and prevents runaway F-actin nucleation during the formation of the first cortical actin meshwork.