Model Chaperones: Small Heat Shock Proteins from Plants

Abstract

32 subunit oligomers. The monomers are defi ned by a conserved α-crystallin domain fl anked by divergent and fl exible N-terminal and C-terminal arms. In higher plants sHSPs have evolved independently of metazoan and bacterial homologs and comprise multiple families of cytosolic proteins, along with proteins targeted to the nucleus, chloroplasts, mitochondria, endoplasmic retic-ulum and peroxisomes. This diversity of sHSPs is unique to land plants and likely arose as a result of their frequent exposure to stress due to their sessile nature. The availability of the high resolution structure of a dodecameric cytosolic class I sHSP from wheat, Ta16.9 (PDB ID: 1GME; 2.7 Å resolution), has facilitated detailed in vitro studies of sHSP chaperone action. A working model proposes that sHSP oligomers dissociate into dimers during heat stress, revealing hydrophobic patches that interact with exposed hydrophobic regions on denaturing substrates, maintaining them in a soluble, folding-competent state. sHSP-substrate complexes are then acted on by ATP-dependent chaperones to restore substrates to their native state. However, much remains to be done to connect this model with the function of the many different sHSPs found in plants. Further genetic and biochemical studies are needed to identify sHSP substrates and to defi ne the mechanism by which sHSPs function, not only during stress, but also during specifi c developmental stages in plants.