The Chaperone Activity and Substrate Spectrum of Human Small Heat Shock Proteins

THE JOURNAL OF BIOLOGICAL CHEMISTRY, VOL. 292, NO. 2, pp. 672–684, DOI 10.1074/jbc.M116.760413

Small heat shock proteins (sHsps) are a ubiquitous family of molecular chaperones that suppress the unspecific aggregation of miscellaneous proteins. Multicellular organisms contain a large number of different sHsps, raising questions as to whether they function redundantly or are specialized in terms of substrates and mechanism. To gain insight into this issue, we undertook a comparative analysis of the 8 major human sHsps on the aggregation of both model proteins and cytosolic lysates under standardized conditions. We discovered that sHsps which form large oligomers (HspB1/Hsp27, HspB3, HspB4/αA-crystallin, and HspB5/αB-crystallin) are promiscuous chaperones, whereas the chaperone activity of the other sHsps is more substrate-dependent. However, all human sHsps analyzed, except HspB7, suppressed the aggregation of cytosolic proteins of HEK293 cells. We identified about 1,100 heat-sensitive HEK293 proteins, 12% of which could be isolated in complexes with sHsps. Analysis of their biochemical properties revealed that most of the sHsp substrates have a molecular mass from 50 to 100 kDa and a slightly acidic pI (5.4-6.8). The potency of the sHsps to suppress aggregation of model substrates is correlated with their ability to form stable substrate complexes: especially HspB1 and HspB5 but also B3 bind tightly to a variety of proteins, while less substrates were detected in complex with the other sHsps, although these were also efficient in preventing the aggregation of cytosolic proteins.

TU München
Helmholtz München
MPI of Neurobiology
MPI of Biochemistry