Hsp42ΔIDD exhibits superior chaperone activity. (A) Coaggregation of MDH-YFP (FRET donor) and MDH labeled with 7-diethylcoumarin-3-carboxylic acid (FRET acceptor) was monitored by FRET (increased acceptor fluorescence) at 47°C in the absence or presence of Hsp42 WT and deletion mutants. (B) Relative proton/deuteron exchange in native or aggregated MDH and MDH coaggregated with Hsp42 WT and Hsp42ΔIDD (at indicated MDH/sHsp ratios) after 30 s incubation in D₂O. Aggregated MDH and MDH–sHsp complexes were formed by incubation for 30 min at 47°C. Error bars denote SD for each point based on three repetitions. All data were corrected for deuteron losses caused by back exchange using a 100% deuterated control. Grey regions could not be detected. (C) HX heat maps of native, aggregated, and Hsp42-complexed states (threefold excess of Hsp42) of the MDH dimer structure (PDB ID: 1MLD) are shown. Peptic peptides are colored according to their exchange behavior (% exchange). Grey regions could not be detected. (D) Bimodal distribution of isotope peaks of indicated MDH peptides derived from MDH–Hsp42 (WT or ΔIDD) complexes. Top: intensity versus m/z diagrams for the indicated peptic MDH fragment after 30 s HX at 30°C. Bottom: fractions of native-like (low HX) and aggregate-like (high HX) populations.