Cyclophilins belong to the family of peptidyl-prolyl cis/trans isomerases 
(PPIases), which are ubiquitous and highly conserved enzymes capable of 
cis/trans isomerizing Xaa-Pro peptide bonds. Members of the CyP40-type 
cyclophilins have originally been described as components of hormone receptor 
complexes. Here, we describe NcCyP41, a CyP40 ortholog from Neurospora crassa, 
its expression in Escherichia coli and subsequent purification. Characterization 
of NcCyP41 reveals that it is a heat shock protein, which is active as a 
cyclosporin A-sensitive PPIase. Affinity chromatography using immobilized 
recombinant NcCyP41 yielded two major NcCyP41-binding proteins: Hsp80 (a Hsp90 
ortholog from N.crassa) and CyPBP37. CyPBP37 has not been described. In 
addition, this is the first record describing an interaction between a member of 
Cyp40-type cyclophilins and of CyPBP37-type proteins, respectively. CyPBP37 
expression is repressed by thiamine and in the stationary phase in N.crassa. 
CyPBP37 is present in different isoforms. The expression of a CyPBP37 ortholog 
in yeast, Thi4p, is diminished in a mutant lacking one of the two CyP40 
orthologs (Cpr7p). In addition, the DeltaCpr7p deletion mutant shows a 
thiamine-dependent growth defect. We conclude that, in yeast, Cpr7p and Thi4p 
interact functionally.