Neurospora crassa FKBP22 is a novel ER chaperone and functionally cooperates with BiP.

FK506 binding proteins (FKBPs) belong to the family of peptidyl prolyl cis-trans isomerases (PPIases) catalyzing the cis/trans isomerisation of Xaa-Pro bonds in oligopeptides and proteins. FKBPs are involved in folding, assembly and trafficking of proteins. However, only limited knowledge is available about the roles of FKBPs in the endoplasmic reticulum (ER) and their interaction with other proteins. Here we show the ER located Neurospora crassa FKBP22 to be a dimeric protein with PPIase and a novel chaperone activity. While the homodimerization of FKBP22 is mediated by its carboxy-terminal domain, the amino-terminal domain is a functional FKBP domain. The chaperone activity is mediated by the FKBP domain but is exhibited only by the full-length protein. We further demonstrate a direct interaction between FKBP22 and BiP, the major Hsp70 chaperone in the ER. The binding to BiP is mediated by the FKBP domain of FKBP22. Interestingly BiP enhances the chaperone activity of FKBP22. Both proteins form a stable complex with an unfolded substrate protein and thereby prevent its aggregation. These results suggest that BiP and FKBP22 form a folding helper complex with a high chaperoning capacity in the ER of Neurospora crassa.