Cytosolic heat-stress proteins Hsp17.7 class I and Hsp17.3 class II of tomato act as molecular chaperones in vivo

Small heat-stress proteins (sHsps) are the most abundant stress-induced pro teins with up to 20 different members in higher plants. In the cytoplasm, t wo different classes call be distinguished. Two cDNA clones from tomato Lyc opersicon peruvianum (L.) Mill., each coding for one of the cytoplasmic sHs p subfamilies, were analyzed with respect to their transcript and protein e xpression, genome organization and chaperone activity. Neither type was pre sent under control conditions but both appeared upon heat stress and in mat ure fruits. Expression of the class II transcript was found to be induced a t slightly lower temperatures than the class I transcript. Protein analysis using class-specific antibodies revealed an identical expression pattern o f both corresponding proteins. Transient expression in an Arabodopsis thali ana (L.) Heynh. cell culture showed that, despite the difference ill their amino acid sequence, both classes are functionally active as chaperones in vivo, as shown by their ability to prevent thermal inactivation of firefly luciferase in a cellular environment.