Clusterin, a binding protein with a molten globule-like region

Clusterin is a heterodimeric glycoprotein found in many tissues of the body and is the most abundant protein secreted by cultured rat Sertoli cells. T he function of clusterin is unknown, but it has been associated with cellul ar injury, lipid transport, apoptosis, and it may be involved in the cleara nce of cellular debris caused by cell injury or death. Consistent with this last idea, clusterin has been shown to bind to a variety of molecules with high affinity including lipids, peptides, and proteins and the hydrophobic probe 1-anilino-8-naphthalenesulfonate (ANS). Given this variety of ligand s, clusterin must have specific structural features that provide the protei n with its promiscuous binding activity. Using sequence analyses, we show t hat clusterin likely contains three long regions of natively disordered or molten globule-like structures containing putative amphipathic alpha -helic es. These disordered regions were highly sensitive to trypsin digestion, in dicating a flexible nature. The effects of denaturation on the fluorescence of the clusterin-ANS complex were compared between proteins with structure d binding pockets and molten globular forms of proteins. Clusterin bound AN S in a manner that was very similar to that of molten globular proteins. Fu rthermore, we found that, when bound to ANS, at least one cleavage site wit hin the protease-sensitive disordered regions of clusterin was protected fr on) trypsin digestion. In addition, we show that clusterin can function as a biological detergent that can solubilize bacteriorhodopsin. We propose th at natively disordered regions with amphipathic helices form a dynamic, mol ten globule-like binding site and provide clusterin the ability to bind to a variety of molecules.