Differential distribution of the cognate and heat-stress-induced isoforms of high Mr cis-trans prolyl peptidyl isomerase (FKBP) in the cytoplasm and nucleoplasm

Wheat root tips express a 73 kDa cognate isoform and a 77 kDa heat-shock-induced isoform of peptidyl prolyl cis-trans isomerase (FK506 binding protein; FKBP) that is part of a chaperone complex with hsp90. The 73 kDa and 77 kDa FKBPs have very similar sequences, differing primarily in the N- and C-terminal 20 amino acids. In order to define the potential functional roles of these proteins, the 73 kDa and 77 kDa FKBPs were localized in root tips using antigen-affinity purified antibodies as a probe. The cognate 73 kDa FKBP is localized in the cytoplasm and appears enriched around the periphery of the early vacuole and vesicles exiting the trans-Golgi. Parallel assays with antibodies directed against tonoplast aquaporin and pyrophosphatase confirmed the association of FKBP with an early vacuole compartment. Sucrose gradient centrifugation analysis of root tip lysates also showed that 73 kDa FKBP is co-fractionated with tonoplast aquaporin and V-ATPase in a light compartment near the top of the gradient. Heat-shock treatment of root tips induces the accumulation of 77 kDa FKBP while the abundance of 73 kDa FKBP remains constant. Quantitative EM immunogold assays of the intracellular distribution of FKBP over an 8 h heat-shock time-course showed that FKBP is initially present in the cytoplasm, but is transported into the nucleus where it accumulates in the nucleoplasm and into specific subnuclear domains. The results of this study show that the intracellular distribution of the high Mr FKBPs in wheat root tips differs at normal and elevated temperatures, indicating different functional roles for the FKBP isoforms.