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The heat-shock protein 90 (Hsp90) molecular chaperones are highly conserved across species. However, their dynamic properties can vary significantly from organism to organism. Here we used high-precision optical tweezers to analyze the mechanical properties and folding of different Hsp90 orthologs, namely bacterial Hsp90 (HtpG) and Hsp90 from the endoplasmic reticulum (ER) (Grp94), as well as from the cytosol of the eukaryotic cell (Hsp82). We find that the folding rates of Hsp82 and HtpG are similar, while the folding of Grp94 is slowed down by misfolding of the N-terminal domain. Furthermore, the domain interactions mediated by the charged linker, involved in the conformational cycles of all three orthologs, are much stronger for Grp94 than for Hsp82, keeping the N-terminal domain and the middle domain in close proximity. Thus, the ER resident Hsp90 ortholog differs from the cytosolic counterparts in basic functionally relevant structural properties. Jahn and Tych et al. report high-precision optical tweezers experiments to analyze the structural and mechanical properties of three orthologs of the heat-shock protein 90 (Hsp90) molecular chaperone family. Hsp90s are highly conserved across species. They find differences in folding rates, domain interactions, and functionally relevant structural stabilities.

The ipsocentric approach to orbital contributions to current density predicts that a triplet state of a 4n π-electron Huckel monocycle should be aromatic, in the sense of supporting a diatropic ring current, and that this current should be carried by just four of the π electrons (three for n=1). The magnetic criterion of aromaticity is thus in agreement with Baird’s classic energy-based prediction of aromatic stabilisation for these species.