Soft and Living Matter Seminar Series: Friday, January 17th, 2020 - To What Extent Does the Physics of Fluids Underly Reflectin’s Dynamically Tunable Structure-Function Relationship?

Event Date: 

Friday, January 17, 2020 - 1:30pm

Event Date Details: 

Distinguished Emeritus Professor of Moleular, Cellular & Developmental Biology

Event Location: 

  • Elings 1601
  • Soft and Living Matter Seminar Series

Reflectin is a cationic, block copolymeric, initially disordered protein that mediates the neuronally triggered, osmotically mediated, dynamic fine-tuning of the color and brightness of light reflected from nanostructured Bragg reflectors in specialized skin cells of squids. Its structure consists of a peptide chain of ca. 350 amino acids, with 6 blocks of unusual and highly conserved (essentially identical) sequence alterating with cationic linkers.

 

Progressive charge-neutralization of reflectin – either by neurotransmitter-activated phosphorylation in vivo, or by pH-titration, genetic engineering or anionic screening of the purified recombinant protein in vitro – drives its condensation, folding and hierarchical assembly to form liquid-liquid phase-separated particles of precisely calibrated size exponentially proportional to the extent of charge-neutralization.  This assembly triggers the osmotic efflux of water from the Bragg lamellae, shrinking their thickness and spacing, while increasing their refractive index contrast – thus dynamically tuning the color while simultaneously increasing the intensity of the reflected light.

 

Our analyses suggest that reflectin’s behavior can be undersood as a consequence of its structure resembling a concatenate of alternating and opposing expansion and contraction springs: Coulombic repulsion of the cationic linkers (expansion) keeps the molecule in an extended and intrinsically disordered state until charge neutralization sufficiently opposes that repulsion, relaxing the stress on the conserved domains to allow the entropic drive encoded in their sequences to trigger condensation and secondary folding (contraction), with the resulting emergence of hydrophobic surfaces and beta structures that facilitate hierarchical assembly. Interestingly, the precise calibration between the extent of charge-neutralization and size of assembly (with consequent calibration between the initiating signal and biological effect) depends on a rapid “dynamic arrest” of assembly.

 

Formally, this dynamic arrest of assembly is determined by the balance of weak, short-range attractive forces and strong long-range repulsive forces, as well understood for many colloidal systems. Professor Fyl Pincus has suggested that a type of Plateau Rayleigh Instability explicable by DLVO Theory may help explain the underlying physics. This is a major focus of our current research.

 

The potential significance of these studies extends beyond reflectin, as proteins associated with Alzheimer’s disease, Parkinson’s disease, diabetes and other degenerative diseases share several key structural features and rearrangements with reflectin.  But in contrast to the case for reflectin, in which these rearrangements are reversible, repeatedly cyclable, non-pathological and key to a physiological process, in the disease-related proteins they are irreversible and pathological.

 

Dan Morse, UCSB