Encysted embryos of the primitive crustacean Artemia franciscana are among the most resistant of all multicellular eukaryotes to environmental stress, in part due to massive amounts of a small heat shock/alpha-crystallin protein (p26) that acts as a molecular chaperone. These embryos also contain very large amounts of the disaccharide trehalose, well known for its ability to protect macromolecules and membranes against damage due to water removal and temperature extremes. Therefore, we looked for potential interactions between trehalose and p26 in the protection of a model substrate, citrate synthase (CS), against heat denaturation and aggregation and in the restoration of activity after heating in vitro. Both trehalose and p26 decreased the aggregation and irreversible inactivation of CS at 43 degrees C. At approximate physiological concentrations (0.4 M), trehalose did not interfere with the ability of p26 to assist in the reactivation of CS after heating, but higher concentrations (0.8 M) were inhibitory. We also showed that CS and p26 interact physically during heating and that trehalose interferes with complex formation and disrupts CS-p26 complexes that form at high temperatures. We suggest from these results that trehalose may act as a "release factor," freeing folding intermediates of CS that p26 can chaperone to the native state. Trehalose and p26 can act synergistically in vitro, during and after thermal stress, suggesting that these interactions also occur in vivo.