Calo, E. | bioRxiv Science

bioRxiv2022

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

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.

biophysics↗

bioRxiv2022

A unified view of LCRs across species

Low complexity regions (LCRs) play a role in a variety of important biological processes, yet we lack a unified view of their sequences, features, relationships, and functions. Here, we use dotplots and dimensionality reduction to systematically define LCR type/copy relationships and create a map of LCR sequence space capable of integrating LCR features and functions. By defining LCR relationships across the proteome, we provide insight into how LCR type and copy number contribute to higher order assemblies, such as the importance of K-rich LCR copy number for assembly of the nucleolar protein RPA43 in vivo and in vitro. With LCR maps, we reveal the underlying structure of LCR sequence space, and relate differential occupancy in this space to the conservation and emergence of higher order assemblies, including the metazoan extracellular matrix and plant cell wall. Together, LCR relationships and maps uncover and identify scaffold-client relationships among E-rich LCR-containing proteins in the nucleolus, and revealed previously undescribed regions of LCR sequence space with signatures of higher order assemblies, including a teleost-specific T/H-rich sequence space. Thus, this unified view of LCRs enables discovery of how LCRs encode higher order assemblies of organisms.

cell biology↗