The major bovine seminal plasma protein, PDC-109 exhibits chaperone-like activity (CLA) against a variety of target proteins. The present studies show that the homologous protein from equine seminal plasma, HSP-1/2 also exhibits CLA and inhibits the thermal aggregation of target proteins such as lactate dehydrogenase, and DTT-induced aggregation of insulin in a concentration-dependent manner. Phosphorylcholine binding inhibited the CLA of HSP-1/2, suggesting that aggregation state of the protein is important for this activity. These results demonstrate that HSP-1/2 functions as a molecular chaperone in vitro, and suggest that it may protect other proteins of equine seminal plasma from unfolding/misfolding or aggregation. These results suggest that homologous proteins from the seminal plasma of other mammals also exhibit CLA, which will be physiologically relevant. (c) 2012 Elsevier Inc. All rights reserved.