Hsp60 Post-translational Modifications: Functional and Pathological Consequences

Abstract

Hsp60 is a chaperone belonging to the Chaperonins of Group I and typically functions inside mitochondria in which, together with the co-chaperonin Hsp10, maintains protein homeostasis. In addition to this canonical role, Hsp60 plays many others beyond the mitochondria, for instance in the cytosol, plasma-cell membrane, extracellular space, and body fluids. These non-canonical functions include participation in inflammation, autoimmunity, carcinogenesis, cell replication, and other cellular events in health and disease. Thus, Hsp60 is a multifaceted molecule with a wide range of cellular and tissue locations and functions, which is noteworthy because there is only one hsp60 gene. The question is by what mechanism this protein can become multifaceted. Likely, one factor contributing to this diversity is post-translational modification (PTM). The amino acid sequence of Hsp60 contains many potential phosphorylation sites, and other PTMs are possible such as O-GlcNAcylation, nitration, acetylation, S-nitrosylation, citrullination, oxidation, and ubiquitination. The effect of some of these PTMs on Hsp60 functions have been examined, for instance phosphorylation has been implicated in sperm capacitation, docking of H2B and microtubule-associated proteins, mitochondrial dysfunction, tumor invasiveness, and delay or facilitation of apoptosis. Nitration was found to affect the stability of the mitochondrial permeability transition pore, to inhibit folding ability, and to perturb insulin secretion. Hyperacetylation was associated with mitochondrial failure; S-nitrosylation has an impact on mitochondrial stability and endothelial integrity; citrullination can be pro-apoptotic; oxidation has a role in the response to cellular injury and in cell migration; and ubiquitination regulates interaction with the ubiquitin-proteasome system. Future research ought to determine which PTM causes which variations in the Hsp60 molecular properties and functions, and which of them are pathogenic, causing chaperonopathies. This is an important topic considering the number of acquired Hsp60 chaperonopathies already cataloged, many of which are serious diseases without efficacious treatment.

Keywords: Hsp60; canonical functions; chaperonin; chaperonopathies; non-canonical functions; post-translation modification.

FIGURE 1

Cartoon representing the human Hsp60 monomer drawn to show some of the known PTM sites and their modifications. Amino acids shown are: Y222 (yellow), Y226 (orange), K396 (cyan), and C237 (blue) in the apical domain (lime); and C442 (light pink) and ATP (red) binding site in the equatorial domain (pale green). Nitration of the much conserved Y222 and Y226, and ubiquitination of K396 in the apical domain might seriously impair chaperoning functions, since this domain is crucial for Hsp10 and client protein binding. S-nitrosylation of C237 was found beneficial for the maintenance of mitochondrial DNA stability, during experimental peritonitis in mice (Suliman et al., 2010). C442 is located near the ATP-binding site in the equatorial domain and its S-guanylation might impair ATPase activity and oligomerization ability. The amino acid sequence of the human Hsp60 was retrieved from the PubMed website (http://www.ncbi.nlm.nih.gov/genbank/), using the accession number NM_002156. The cartoon was drawn using SWISS-MODEL (http://swissmodel.expasy.org/) accessible via the ExPASy web server (http://www.expasy.org/); and was visualized and modified by PyMol (http://www.pymol.org).

FIGURE 2

Hsp60 post-translational modifications. Linear representation of human Hsp60 with the N-terminal, 26 amino acids-long, mitochondrial import sequence (MIS) to the left; the two segments of the equatorial domain in gray (residues 30–157 and 434–548 in the Hsp60 full-length sequence), containing the ATP-binding pocket; the two segments of the intermediate domain in light gray (residues 158–214 and 402–433), connecting the equatorial and the apical domains; and the apical domain in dark gray (residues 215–401), involved in substrate-recruitment and co-chaperonin binding. On the left of the figure all reported PTMs are indicated with a letter with a color code: Phosphorylation (P) in orange, Acetylation (A) in green, Ubiquitination (U) in red, Succynilation (Sc) in light blue, Methylation (M) in brown, S-Nitrosylation (s-N) in blue, S-guanylation (s-G) in magenta, and Nitration (N) in dark purple. Along the linear representation of the Hsp60, aligned with each letter, the residues involved in the corresponding PTM are indicated with the same color as that of the pertinent modification. The data were obtained from the PTM database PhosphoSitePlus (http://www.phosphosite.org) and from the scientific literature.