The recent explosion of cryo-EM structures has emphasized the stepwise assembly of stable biomolecular machines with fixed compositions. In contrast, recent advances in single-molecule imaging in vitro and in live cells reveal a strikingly different picture: Biological complexes are far more dynamic and transient than previously appreciated. Rather than persisting as stable machines, their functions emerge from fluid, short-lived assemblies, whose lifetimes and outputs are governed by the conformational and compositional kinetics of their components. This paradigm shift, from stable machines to fluid assemblies, offers a powerful framework for understanding gene regulation, cellular proofreading, checkpoint control, and rapid adaptation to environmental cues. This revamped image is perhaps most clearly emerging in the context of RNA:protein (RNP) complex assembly.
The current presentation will illustrate this developing perspective with case studies from two distinct biological contexts: bacterial transcription factors whose function depends on dynamic exchange; and the kinetically-controlled assembly of higher-order structures in mammalian cells termed membraneless organelles or RNP granules. I will also examine the broader implications of this framework, including how regulatory inputs can rewire molecular function by altering interaction kinetics rather than binding affinity or structure. By reframing molecular cell biology through the lens of kinetic behavior and spatiotemporal organization, this presentation will provide a unifying conceptual foundation across the molecular and cell biology communities.