PPIases are ubiquitous in living organisms. While three families of PPIases, cyclophilin (CyP), FK506 binding protein (FKBP) and parvulin (Pvn), have been studied in detail in Eukarya and Bacteria (eubacteria), little is known about archaeal PPIases. Among 13 cyclophilins found in Archaea, only Halobacterium cyclophilin (HbsCyP19) has been characterized. This is a cyclosporin A (CsA) sensitive CyP with a molecular weight of 19.4 kDa. The PPIase activity and CsA sensitivity of HbsCyP19 is higher at higher salt concentration in the medium. No parvulin except a homolog in Cenarchaeum symbiosum has been found in Archaea. Two types of FKBPs, 26-30 kDa long-type and 17-18 kDa short-type FKBP, have been found in Archaea. Up to date, 12 short-type FKBPs and 18 long-type FKBPs have been known. The short-type FKBPs and N-terminal sequences of the long-type FKBPs are similar to each other and show homology to human FKBP12 (HsFKBP12). However, they have two insertion sequences in the regions corresponding to bulge and flap loops of HsFKBP12. The long-type archaeal FKBPs have additional ca. 100 amino-acid sequences at their C-terminal regions. A short-type archaeal FKBP from Methanothermococcus thermolithotrophicus has not only a PPIase activity but also a chaperone-like activity, which includes protein refolding and aggregation suppressing activities with regard to protein folding intermediates. Mutational analysis revealed that this chaperone-like activity was independent of the PPIase activity, and that the insertion sequence in the region corresponding to the flap seemed to be important. Three-dimensional structure of this FKBP showed that the insertion in the flap makes a domain which has a hydrophobic surface. Coexpression of aggregation prone proteins with these archaeal FKBPs were shown to improve their expression in soluble fraction in Escherichia coli. Fusion protein of the archaeal FKBP and an aggregation prone protein also show improved expression of the latter in E. coli.