Post-translational modifications of Hsp70 family proteins: Expanding the chaperone code

Abstract

Cells must be able to cope with the challenge of folding newly synthesized proteins and refolding those that have become misfolded in the context of a crowded cytosol. One such coping mechanism that has appeared during evolution is the expression of well-conserved molecular chaperones, such as those that are part of the heat shock protein 70 (Hsp70) family of proteins that bind and fold a large proportion of the proteome. Although Hsp70 family chaperones have been extensively examined for the last 50 years, most studies have focused on regulation of Hsp70 activities by altered transcription, co-chaperone "helper" proteins, and ATP binding and hydrolysis. The rise of modern proteomics has uncovered a vast array of post-translational modifications (PTMs) on Hsp70 family proteins that include phosphorylation, acetylation, ubiquitination, AMPylation, and ADP-ribosylation. Similarly to the pattern of histone modifications, the histone code, this complex pattern of chaperone PTMs is now known as the "chaperone code." In this review, we discuss the history of the Hsp70 chaperone code, its currently understood regulation and functions, and thoughts on what the future of research into the chaperone code may entail.

Keywords: AMPylation; BiP; Hsc70; Hsp70; PTM; Ssa1; acetylation 70 kilodalton heat shock protein (Hsp70); chaperone; methylation; molecular chaperone; phosphorylation; post-translational modification; post-translational modification (PTM); protein AMPylation; protein methylation; protein phosphorylation.

Figure 1.

The post-translational modifications of mammalian Hsp70, Hsc70, and BiP and yeast Ssa1–4. Shown is a domain representation of the Hsp70 family members, with detected PTMs marked with appropriate residue numbers. PTMs are labeled as follows: phosphorylation in red, ubiquitination in green, acetylation in yellow, methylation in cyan, ADP-ribosylation in purple, and AMPylation in dark blue.

Figure 2.

Locations of PTMs on the Hsp70 structure. PTMs were mapped onto predicted structural models created by SWISS-MODEL based on Protein Data Bank entry 2KHO for each Hsp70 isoform (192). PTMs are colored as in Fig. 1, except sites of multiple modification are labeled brown.

Figure 3.

Conservation of Hsp70 PTMs and surrounding sequence between Hsp70 isoforms. Alignment was build using human Hsp70, Hsc70, and BiP as well as S. cerevisiae Ssa1, Ssa2, Ssa3, and Ssa4 sequences. Sequences were aligned using ClustalX (193), and sites of PTM were labeled as in Fig. 2.

Figure 4.

The hallmarks of Hsp70 regulation. This illustration encompasses six regulatory processes (outside wheel) that are affected by the post-translational modification of Hsp70: co-chaperone binding, expression, client binding, localization, ATPase activity, and oligomerization. Most of these processes are interdependent (e.g. ATPase activity and client binding).