Peeking from behind the veil of enigma: emerging insights on small heat shock protein structure and function

This is a short paper on new ways to think about the structure and function of small heat shock proteins (sHSPs), perhaps the most enigmatic family among protein chaperones. The goal is to incorporate new observations regarding the disordered regions of small heat shock proteins (sHSPs) into the large body of structural information on the conserved structural alpha-crystallin domains (ACD) that define the sHSP family. Disordered regions (N-terminal region and C-terminal region or NTR and CTR, respectively) represent over 50% of the sHSP sequence space in the human genome and are refractory to traditional structural biology approaches, posing a roadblock on the path towards a mechanistic understanding of how sHSPs function. A model in which an ACD dimer serves as a template that presents three grooves into which other proteins or other segments of sHSPs can bind is presented. Short segments within the disordered regions are observed to bind into the ACD grooves. There are more binding segments than there are grooves, and each binding event is weak and transient, creating a dynamic equilibrium of tethered and untethered disordered regions. The ability of an NTR to be in dynamic equilibrium between tethered/sequestered and untethered states suggests several mechanistic alternatives that need not be mutually exclusive. New ways of thinking about (and approaching) the intrinsic properties of sHSPs may finally allow the veil of enigma to be removed from sHSPs.