Paradoxical inhibition of protein aggregation and precipitation by transglutaminase-catalyzed intermolecular cross-linking.

Cross-linking of proteins catalyzed by tissue transglutaminase has been suggested to play key roles in a variety of cellular events, including cell apoptosis and human pathogenesis (e.g. polyglutamine and Alzheimer diseases). It has often been suggested that tissue transglutaminase enhances aggregation and precipitation of damaged or pathogenic proteins. To ascertain whether this is accurate, we investigated the effects of tissue transglutaminase-catalyzed modulation on the aggregation of structurally damaged and unfolded proteins. Our results indicated that the aggregation and precipitation of some unfolded proteins were inhibited by transglutaminasecatalyzed reaction, although the effect was strongly dependent upon the target protein species. To elucidate the molecular events underlying the inhibitory effect, extensive analysis was performed with regard to reduced beta-lactoglobulin using a number of techniques, including chromatography and spectroscopy. The results indicated that cross-linking yields high molecular weight soluble polymers but inhibits the growth of insoluble aggregates. The cross-linked beta-lactoglobulin retained stable secondary structures with a hydrophobic core. We concluded that the transglutaminase-catalyzed intermolecular cross-linking did not necessarily enhance protein aggregation but could sometimes have a suppressive effect. The results of the present study suggested that tissue transglutaminase modifies aggregation and deposition of damaged or pathogenic proteins in vivo in a wide variety of manners depending on the target protein species and solution conditions.