Crystal structure of the three FK506 binding protein domains of wheat FKBP73: Evidence for a unique wFK73-2 domain

Here we describe the crystal structure of the Nterminal domain of the FK506-binding protein (FKBP) from wheat (wFKBP73), which is the first structure presenting three FK domains (wFK73-1 , wFK73-2 and wFK73-3). The crystal model includes wFK73-2 and wFK73-3 domains and only part of the wFK73-1 domain. The wFK73-1 domain is responsible for binding FK506 and for peptidyl prolyl cis/trans isomerase (PPIase) activity, while the wFK73-2 and wFK73-3 domains lack these activities. A structure-based sequence comparison demonstrated that the absence of a large enough hydrophobic pocket important for PPIase activity, and of the conserved residues necessary for drug binding in the wFK73-2 and wFK73-3 domains explains the lack of these activities in these domains. Sequence and structural comparison between the three wFKBP73 domains suggest that the wFK73-2 domain is the most divergent. A structural comparison of the FK domains of wFKBP73 with other FKBPs containing more than one FK domain, revealed that while the overall architecture of each of the three FK domains displays a typical FKBP fold, their relative arrangement in space is unique and may have important functional implications. We suggest that the existence of FKBPs with three FK domains offers additional interactive options for these plant proteins enlarging the overall regulatory functions of these proteins.