Functional Mode of NtHSP17.6, a Cytosolic Small Heat-Shock Protein from Nicotiana tabacum

Abstract

Small heat-shock proteins (sHsps) are ubiquitous stress proteins with molecular chaperone activity. They share characteristic homology with the α-crystallin protein of the mammalian eye lens as well as being ATP-independent in their chaperone activity. We isolated a clone for a cytosolic class I sHsp, NtHSP17.6, from Nicotiana tabacum, and analyzed its functional mode for such activity. Following its transformation into Escherichia colt and its over-expression, NtHSP17.6 was purified and examined in vitro. This purified NtHSP17.6 exhibited typical chaperone activity in a light-scattering test. It was enable to protect a model substrate, firefly luciferase, from heat-induced aggregation. Non-denaturing PAGE showed that NtHSP17.6 formed a dodecamer in its native conformation, and was bound to its substrate under heat stress. A labeling test with bis-ANS indicated that this binding might be linked to newly exposed hydrophobic sites of the NtHSP17.6 complexes during heat shock. Based on these data, we suggest that NtHSP17.6 is a molecular chaperone that functions as a dodecamer in a heat-induced manner.