Title:   A protein that acts like a paper ball

Abstract:  Disordered proteins display an intriguing mix of entropic freedom and weak transient interactions that underlie their biological roles. Motivated to understand that interplay,  and building upon our group's experience in using polymer elasticity to gain insight into polymeric structure and behavior, we carried out single-molecule stretching measurements on a model protein construct derived from the disordered tail domains of neurofilament proteins. We particularly measured the response of the construct to sudden changes in applied tension. Unexpectedly, we found that the construct displays an anomalous (‘glassy’) mechanical response: a one-step change in the applied force causes a slow, logarithmic response in the chain extension. Further, the disordered chain exhibits a distinct memory effect (the Kovacs effect), in which the chain ‘remembers’ changes in force that occurred tens of seconds prior. To interpret these findings, we turned to recent studies that showed similar dynamics in a completely different system—the slow dynamics of crumpled paper balls. I will discuss how the insight from crumpled paper gave us clues into the molecular processes at work in the protein system, ultimately revealing that the mechanisms of kinetics in the disordered chain differ from those typically invoked to explain analogous behavior in structured proteins.

Speaker's Bio: Professor Omar A. Saleh is a physicist and materials scientist with broad expertise in biomolecular and polymer science. Saleh received his B.S. in Physics from MIT in 1997, and his Ph.D. in Physics from Princeton in 2003. His graduate studies were supported by a Hertz Fellowship. He was a postdoctoral fellow at the École Normale Supérieure in Paris, France, where he developed single-molecule experimental techniques to study motor protein/DNA interactions. He came to UC Santa Barbara in 2005, where he is now a full professor in the Materials Department, with a minority appointment in the Biomolecular Science and Engineering (BMSE) Program. He served as Director of the BMSE program from 2013 to 2017 and was an elected member of the Executive Committee of the Division of Biological Physics of the American Physical Society from 2013 to 2016.  His research is focused on the molecular physics underlying biological systems, with particular experience in nucleic acids, protein/DNA interactions, motor proteins, biomolecular elasticity, and self-assembled biomolecular systems. His research achievements were recognized by an NSF CAREER award in 2008, by a Bessel Research Award from the Alexander von Humboldt Society in 2017, and by his selection as a Fellow of the American Physical Society in 2019 by the Division of Biological Physics.