Marina Feric, Pennsylvania State University - Department of BMB

Title: Engineering cellular organization: from molecules to condensates

Abstract:

Many membrane-less cellular complexes assemble via phase separation. Cellular components de-mix from their surroundings to form liquid-like phases called biomolecular condensates. How the cell regulates these processes and how the emergent material properties contribute to function, both in health and disease, are poorly understood. Here, I explore the role of phase separation in organizing the transcriptionally active nucleolus in large frog eggs and mitochondrial transcriptional condensates in the context of aging. I found that the frog has evolved a unique nuclear actin scaffold that mechanically stabilizes its nuclear condensates, including nucleoli, against gravity. Moreover, the three functional nucleolar compartments rearrange into prominent drop-within-a-drop morphologies upon actin disruption, indicating that the nucleolus behaves a multi-phase droplet whose organization is governed by differences in surface tension, thereby coordinating the flow of gene expression. Phase separation also occurs within mitochondria. Mitochondria contain their own genome (mtDNA) which is packaged by proteins to form membrane-less nucleoprotein complexes called mt-nucleoids. The main packaging protein, TFAM, can phase separate on its own in vitro, and with the addition of mtDNA, forms heterogenous, viscoelastic droplets consistent with the phase behavior of mt-nucleoids observed in vivo. Consistently, the core mt-transcription machinery self-assembles into condensates in vitro, where transcription occurs with dampened rates and with the emergence of vesicle-like morphologies. The mt-transcription machinery thus serves as a model system for studying the biophysics and kinetics of transcriptional condensates. These results highlight the ubiquity of phase separation in gene regulation and the various mechanisms by which cells can harness phase separation to organize genomes.