The small heat-shock chaperone protein, α-crystallin, does not recognise stable molten globule states of cytosolic proteins

Abstract

The small heat-shock protein (sHsp), α-crystallin, acts as a molecular chaperone by interacting with destabilised ‘substrate’ proteins to prevent their precipitation from solution under conditions of stress. α-Crystallin and all sHsps are intracellular proteins. Similarly to other chaperones, the ‘substrate’ protein is in an intermediately folded, partly structured molten globule state when it interacts and complexes with α-crystallin. In this study, stable molten globule states of the cytosolic proteins, γ-crystallin and myoglobin, have been prepared. Within the lens, γ-crystallin naturally interacts with α-crystallin and myoglobin and α-crystallin are present together in muscle tissue. The molten globule states of γ-crystallin and myoglobin were prepared by reacting γ-crystallin with glucose 6-phosphate and by removing the haem group of myoglobin. Following spectroscopic characterisation of these modified proteins, their interaction with α-crystallin was examined by a variety of spectroscopic and protein chemical techniques. In both cases, there was no interaction with α-crystallin that led to complexation. It is concluded that α-crystallin does not recognise stable molten globule states of cytosolic ‘substrate’ proteins and only interacts with molten globule states of proteins that are on the irreversible pathway towards an aggregated and precipitated form.