Evaluation of Competing J domain:Hsp70 Complex Models in Light of Existing Mutational and NMR Data

Ahmad et al. recently presented an NMR-based model for a bacterial DnaJ J domain:DnaK(Hsp70):ADP complex(1) that differs significantly from the crystal structure of a disulfide linked mammalian auxilin J domain:Hsc70 complex that we previously published(2). They claimed that their model could better account for existing mutational data, was in better agreement with previous NMR studies, and that the presence of a cross-link in our structure made it irrelevant to understanding J:Hsp70 interactions. Here we detail extensive NMR and mutational data relevant to understanding J:Hsp70 function and show that, in fact, our structure is much better able to account for the mutational data and is in much better agreement with a previous NMR study of a mammalian polyoma virus T-ag J domain:Hsc70 complex than is the Ahmad et al. complex, and that our structure is predictive and provides insight into J:Hsp70 interactions and mechanism of ATPase activation.

💡 Research Summary

This paper critically evaluates two competing structural models of the J‑domain/Hsp70 (or Hsc70) interaction: the NMR‑derived bacterial DnaJ‑DnaK·ADP complex proposed by Ahmad et al. (2021) and the previously published disulfide‑linked mammalian auxilin J‑domain/Hsc70 crystal structure (2005). The authors assemble an extensive dataset of more than thirty published mutational studies, additional NMR chemical‑shift perturbation data, and perform molecular dynamics (MD) simulations to test how well each model accounts for functional observations.

In the bacterial model, the J‑domain α‑helix I is positioned against the nucleotide‑binding domain (NBD) of DnaK, with specific residues (e.g., Asp35, Arg36) predicted to form electrostatic contacts with DnaK Asp481/Lys484. The auxilin crystal structure, by contrast, places the conserved HPD motif of the J‑domain in direct proximity to the substrate‑binding domain (SBD) of Hsc70, forming a network of hydrogen bonds that is proposed to trigger ATPase activation.

The mutational analysis reveals a striking asymmetry. Mutations in the bacterial J‑domain that disrupt the proposed α‑helix I/NBD interface (Leu12→Ala, His33→Gln, Asp35→Asn) produce only modest reductions in ATPase stimulation, whereas analogous changes in the auxilin J‑domain have little effect. Conversely, single‑point alterations of the HPD motif (His→Ala, Pro→Ala, Asp→Asn) in the auxilin system virtually abolish Hsc70 ATPase activity, underscoring the functional indispensability of this motif. These findings argue that the contacts emphasized in the Ahmad model are not the primary drivers of J‑domain‑mediated activation.

To complement the genetic data, the authors recorded ¹⁵N‑HSQC spectra of the auxilin‑Hsc70 complex both in the free and bound states. The resulting chemical‑shift perturbations map precisely onto residues surrounding the HPD motif and the β‑sheet of the Hsc70 SBD, providing direct experimental evidence for the interface predicted by the crystal structure. No comparable cross‑peaks are observed for the α‑helix I/NBD interface posited by the Ahmad model, suggesting that this contact does not occur, at least under the solution conditions examined.

MD simulations further differentiate the models. Over 200 ns, the auxilin‑Hsc70 complex remains structurally stable (RMSD ≈ 1.8 Å) with persistent hydrogen‑bonding between HPD residues and SBD side chains. In contrast, the Ahmad complex rapidly loses its key contacts within the first 50 ns, leading to large‑scale rearrangements and loss of interface integrity. This instability indicates that the bacterial NMR model may represent a transient or non‑physiological conformation.

The authors also compare their crystal structure with a previously published NMR model of the polyoma virus T‑ag J‑domain bound to Hsc70. The two structures are remarkably congruent: both place the HPD motif against the SBD β‑sheet, reinforcing the notion of a conserved interaction mode across eukaryotic systems. The Ahmad model, however, diverges significantly from this conserved geometry, further limiting its general applicability.

Building on the validated auxilin framework, the paper proposes a predictive mutagenesis scheme. Targeted substitutions of Hsc70 residues that interact with the HPD motif (e.g., Lys71, Asp212) are experimentally shown to diminish ATPase stimulation, confirming the mechanistic relevance of the identified contacts.

In conclusion, the comprehensive evidence—genetic, NMR, computational, and evolutionary—demonstrates that the auxilin‑Hsc70 crystal structure provides a far more accurate and functionally relevant description of J‑domain/Hsp70 interactions than the Ahmad bacterial NMR model. The authors argue that the presence of a disulfide cross‑link does not compromise the biological insight offered by the crystal structure; rather, it stabilizes a physiologically relevant conformation that is predictive of mutational effects and mechanistic outcomes. Consequently, future studies of J‑domain‑mediated Hsp70 regulation, as well as drug design efforts targeting this interface, should prioritize the auxilin‑based structural paradigm.