TCOF1 is a single-component scaffold of the nucleolar fibrillar center

Abstract

Many of the biological structures that exist across the tree of life are built on self-interacting scaffolds, from the actin cytoskeleton to the collagen extracellular matrix. Intracellular membraneless organelles, such as the nucleolus, are biological structures consisting of hundreds of dynamically interacting components, yet it is unclear whether the underlying organization of these complex assemblies can be scaffolded by such self-interacting components. Here, we show that TCOF1 is a single-component scaffold of the nucleolar fibrillar center (FC), based on thermodynamics of its assembly in cells, as well as sufficiency and loss-of-function experiments. TCOF1 is necessary for the formation of the FC, and defines the FC through assembly mediated by homotypic interactions of its Serine/Glutamate (S/E)-rich low-complexity regions (LCRs). Ultimately, introduction of TCOF1 into a species that lacks the FC is sufficient to form an FC-like nucleolar subcompartment. Thus, we demonstrate how a single protein component can explain the formation and evolution of a complex biological structure.