Molecular chaperones play a crucial role in proteostasis (protein homeostasis) by balancing protein quality control, folding and turnover. They therefore have the ability and malleability to bind to any protein and detect if it is misfolded.
To date, there are ~180 dedicated chaperones found in the human body, which begs the question: How does such a relatively small number of chaperones maintain proteostasis? Moreover, once a chaperone binds to a protein how does it decide what to do with it? The answer to these questions lies in protein-protein interactions (PPIs). Researchers are finding that certain PPIs coordinate multiple chaperones into organized, functional complexes as well as “handoff” proteins between them. Other cellular pathways such as those that control trafficking such as degradation are also linked to chaperones by PPIs.
The various PPIs of the chaperone network not only have a wide range of affinity values (nanomolar to micromolar) but include several distinct domain motifs. These include J domains, zinc fingers and tetratricopeptide repeats. By having these shared motifs, the chaperones often compete for the same interactions. This means that a collection of PPIs draws together the chaperones and target proteins to create a subnetwork that govern protein quality control.
Examples in the chaperone network
One important function of chaperones is to prevent newly synthesized polypeptide chains and assembled subunits from aggregating into non-functional structures. For this reason, the largest family of chaperones are “heat shock proteins” (Hsps)1. When cells are exposed to stress factors such as heat the likelihood for proteins to aggregate increases. Under these circumstances Hsps bind to proteins and protect them from aggregating. Hsp70, for example, can interact with proteins reversibly to their hydrophobic regions to reduce aberrant (i.e. non-native) contacts. Interestingly, Hsps are conserved through all kingdoms of life, which suggests that they are an ancient method of protecting proteasomes.
Chaperones do not just prevent aggregation however, activities such as “folding” are also carried out by groups of chaperones. This type of activity is usually powered by ATP hydrolysis which requires a co-ordinated effort from different groups of Hsps and co-chaperones. It has also been observed that some proteins are “handed off” from one chaperone to another. Steroid hormone receptors (SHRs) are an example of this, SHRs are shuttled between Hsp70 and Hsp90 through the action of a co-chaperone, HOP (Hsp70-Hsp90 organizing protein)2.
Why chaperones are hard to study
Due to the importance of PPIs dictating chaperone functionality, methods to target specific interactions have been sought after. One major problem however is that the physical interactions between chaperones are weak. Current methods require breaking down chaperones to constituent parts or fixing them to a surface. This has had limited success since there are relatively few platforms at researchers’ disposal.
However, a novel biophysical method, Microfluidic Diffusional Sizing (MDS), has been developed to study PPIs and chaperones. In April 2019, Sheidt et al.3 published a paper detailing how two chaperones; Cluterin and Brichos domain, were shown to to inhibit the elongation of Aβ fibrils. Historically, it has been difficult to interpret the macroscopic physical observations of experiments that deal with Aβ formation in Alzheimer’s disease. But with MDS this is starting to change.
Similarly, Wright et al.4 used MDS technology to study the oligomerization of SBD641, the substrate binding subdomain of human Hsp70. In the paper, the authors noted that MDS offers a new method to test monodisperse solutions of isolated components.
These observations are a small representation of the dynamic, interconnected web of chaperones that coordinate functions and share molecular information to maintain proteostasis. If you are interested in learning more about how MDS technology is changing the way researchers are studying protein–protein interactions click here.
(1) Hartl, F. U.; Bracher, A.; Hayer-Hartl, M. Molecular chaperones in protein folding and proteostasis. Nature. 2011, 475, 324−332.
(2) Pratt, W. B.; Toft, D. O. Regulation of signaling protein function and trafficking by the hsp90/hsp70-based chaperone machinery. Experimental Biology & Medicine. 2003, 228, 111−133.
(3) Scheidt, Tom, Urszula Łapińska, Janet R. Kumita, Daniel R. Whiten, David Klenerman, Mark R. Wilson, Samuel IA Cohen et al. "Secondary nucleation and elongation occur at different sites on Alzheimer’s amyloid-β aggregates." Science advances. 2019, 5 : eaau3112.
(4) Wright, Maya A., Francesco A. Aprile, Mathias MJ Bellaiche, Thomas CT Michaels, Thomas Müller, Paolo Arosio, Michele Vendruscolo, Christopher M. Dobson, and Tuomas PJ Knowles. Cooperative Assembly of Hsp70 Subdomain Clusters. Biochemistry. 2018, 57, 3641-3649.