Abstract
The small heat shock protein, human HspB2, also known as Myotonic Dystrophy Kinase Binding Protein (MKBP), specifically associates with and activates Myotonic Dystrophy Protein Kinase (DMPK), a serine/threonine protein kinase that plays an important role in maintaining muscle structure and function. The structure and function of HspB2 are not well understood. We have cloned and expressed the protein in E.coli and purified it to homogeneity. Far-UV circular dichroic spectrum of the recombinant HspB2 shows a β-sheet structure. Fluorescence spectroscopic studies show that the sole tryptophan residue at the 130th position is almost completely solvent-exposed. Bis-ANS binding shows that though HspB2 exhibits accessible hydrophobic surfaces, it is significantly less than that exhibited by another well characterized small HSP, αB-crystallin. Sedimentation velocity measurements show that the protein exhibits concentration-dependent oligomerization. Fluorescence resonance energy transfer study shows that HspB2 oligomers exchange subunits. Interestingly, HspB2 exhibits target protein-dependent chaperone-like activity: it exhibits significant chaperone-like activity towards dithiothreitol (DTT)-induced aggregation of insulin and heat-induced aggregation of alcohol dehydrogenase, but only partially prevents the heat-induced aggregation of citrate synthase, co-precipitating with the target protein. It also significantly prevents the ordered amyloid fibril formation of α-synuclein. Thus, our study, for the first time, provides biophysical characterization on the structural aspects of HspB2, and shows that it exhibits target protein-dependent chaperone-like activity.
Highlights
Myotonic Dystrophy Protein Kinase Binding Protein (MKBP) associates with and activates Myotonic Dystrophy Protein Kinase (DMPK), a serine/threonine protein kinase that plays an important role in maintaining muscle structure and function [1]
We studied the secondary structure of HspB2 by far-UV Circular Dichroism (CD) spectroscopy
MKBP/HspB2 appears to be an important player in regulating the homeostasis of the kinase, DMPK which has pleotrophic functions such as skeletal muscle integrity, cardiac muscle atrioventricular conduction and ion-channel gating to mention a few [37]
Summary
Myotonic Dystrophy Protein Kinase Binding Protein (MKBP) associates with and activates Myotonic Dystrophy Protein Kinase (DMPK), a serine/threonine protein kinase that plays an important role in maintaining muscle structure and function [1]. Earlier studies implicate an important role for HspB2 in cellular function and pathology. HspB2 is known to protect myocardium from ischemia and help in recovering from ischemic stress during reperfusion. It is important in ATP turnover in mouse heart [12]. HspB2 has been found in the senile plaques of Alzheimer disease [14] Though these studies clearly indicate the importance of HspB2 in various cellular functions and pathology, its structural and functional aspects have not yet been addressed. Our study for the first time sheds light on the structural aspects of HspB2 and shows that it exhibits molecular chaperone property in preventing amorphous as well as amyloid aggregation of proteins. Unlike other mammalian sHsps characterized so far, it exhibits unique target protein-dependent chaperone activity
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