Abstract: Some proteins exist at supersaturating levels in cells, but remain soluble due to a large nucleation barrier that makes the appearance of aggregates de novo improbable. We have discovered that cells modulate nucleation barriers to control when and where such phase transitions occur, thereby producing switch-like changes in protein activity. Evidence of this form of protein regulation pervades the proteomes of complex eukaryotes and is fundamental to human health and disease. On the one hand, the aggregation of supersaturated proteins is heavily implicated in ALS, Alzheimer’s, Parkinson’s, and Huntington’s diseases. On the other hand, nucleation-limited aggregation has been proposed as a mechanism for cellular memory and signal transduction. We seek to explore the breadth of biological effects mediated by nucleation-limited protein aggregation and to decipher the rules that govern it. We do so by investigating supersaturable proteins from two extremes of conformation space: intrinsically disordered, low sequence complexity regions commonly involved in gene regulation, and globular death domains involved with mammalian innate immunity and programmed cell death. Our findings with these proteins establish a general role for nucleation-limited aggregation in cell fate determination. First, we have discovered that low sequence complexity proteins commonly undergo environmentally-responsive phase transitions in budding yeast and can act as epigenetic determinants of multicellularity. We have further found that the different growth forms produced by the supersaturated or post-nucleated states of the proteins exhibit frequency-dependent fitness interactions that drive primitive metabolic divisions of labor. Second, we have discovered that multiple proteins belonging to the mammalian death domain superfamily undergo prion-like, nucleated phase transitions that functionally commit cells to inflammatory responses and programmed cell death. Finally, we have developed a powerful new method – Distributed Amphifluoric FRET (DAmFRET) -- that enables high throughput detection and quantification of nucleation barriers and saturating concentrations that govern these phenomena in vivo. DAmFRET has greatly streamlined our discovery of supersaturable proteins and is confirming a driving role for nucleation-limited aggregation in diverse physiological and pathological processes.

Bio: After obtaining his PhD in Biology with Susan Lindquist at MIT, Dr. Halfmann transitioned directly to an independent research career at UT Southwestern Medical Center. As an independent postdoctoral fellow there, he secured NIH funding to build a research program that uses yeast genetics, molecular biology, and biochemistry to explore the contributions of protein aggregation to gene regulation and phenotypic heterogeneity. He discovered that self-propagating protein aggregates known as prions function to enforce cell fate commitment in organisms ranging from budding yeast to humans. In 2015 he became an Assistant Investigator at the Stowers Institute in Kansas City, MO, where his lab investigates how nucleation-limited protein phase transitions drive both function and dysfunction in biological systems, including signal transduction, cellular memories, and aging.