Structural and functional characteristics of clusterin protein in the testis

Clusterin is a heterodimeric glycoprotein that is produced constituitively by rat Sertoli cells and has been found in several tissues and species. The function of clusterin is unknown but it has been associated with cellular injury, lipid transport and apoptosis. Its localization in the testis suggests that clusterin plays a role in spermatogenesis. Here, work regarding structural features of clusterin protein and the characterization of clusterin knock-out mice are described.;Clusterin has been shown to bind to a variety of molecules including lipids, peptides and proteins. We hypothesized that clusterin has specific structural features that provide the protein with its promiscuous binding activity. We show that clusterin contains three long regions of natively disordered structure containing putative amphipathic alpha-helices. These disordered regions were sensitive to trypsin digestion due to their flexible nature. Urea titration profiles of clusterin-ANS complexes were found to be more similar to molten globular proteins than proteins with well structured binding pockets or clefts. ANS binding protects one site within the disordered regions from trypsin digestion. Clusterin also had detergent-like characteristics and maintained the solubility of bacteriorhodopsin in aqueous buffer.;The association of clusterin with apoptosis and cellular injury suggests that clusterin protects cells from apoptosis or may be involved in tissue remodeling. To investigate the role of clusterin in the testis we analyzed spermatogenesis of clusterin knock-out (cluKO) and the response to injury caused by MAA toxicity and heat exposure. CluKO mice were fertile and had normal spermatogenesis with the exception of incomplete spermiation after stage VIII. No differences in morphology or apoptosis were seen between the cluK0 and clusterin wild type (cluWT) mice post-MAA treatment. In contrast, apoptosis was delayed in the cluWT mice compared to the cluK0 mice following heat exposure and dying cells were removed more rapidly in the cluWT mouse suggesting a role in tissue remodeling.;We propose that clusterin functions as a biological detergent to aid in the removal of cellular debris during tissue remodeling.