The plant co-chaperones FK506-binding proteins (FKBPs) are peptidyl prolyl 
cis-trans isomerases that function in protein folding, signal transduction and 
chaperone activity. We report the characterization of the Arabidopsis large 
FKBPs ROF1 (AtFKBP62) and ROF2 (AtFKBP65) expression and protein accumulation 
patterns. Transgenic plants expressing ROF1 promoter fused to GUS reporter gene 
reveal that ROF1 expression is organ specific. High expression was observed in 
the vascular elements of roots, in hydathodes and trichomes of leaves and in 
stigma, sepals, and anthers. The tissue specificity and temporal expression of 
ROF1 and ROF2 show that they are developmentally regulated. Although ROF1 and 
ROF2 share 85% identity, their expression in response to heat stress is 
differentially regulated. Both genes are induced in plants exposed to 37 degrees 
C, but only ROF2 is a bonafide heat-stress protein, undetected when plants are 
grown at 22 degrees C. ROF1/ROF2 proteins accumulate at 37 degrees C, remain 
stable for at least 4 h upon recovery at 22 degrees C, whereas, their mRNA level 
is reduced after 1 h at 22 degrees C. By protein interaction assays, it was 
demonstrated, that ROF1 is a novel partner of HSP90. The five amino acids 
identified as essential for recognition and interaction between the mammalian 
chaperones and HSP90 are conserved in the plant ROF1-HSP90. We suggest that 
ROF/HSP90 complexes assemble in vivo. We propose that specific complexes 
formation between an HSP90 and ROF isoforms depends on their spatial and 
temporal expression. Such complexes might be regulated by environmental 
conditions such as heat stress or internal cues such as different hormones.