The role of the molecular chaperone CCT in protein folding and mediation of cytoskeleton-associated processes: implications for cancer cell biology

Abstract

The chaperonin-containing tailless complex polypeptide 1 (CCT) is required in vivo for the folding of newly synthesized tubulin and actin proteins and is thus intrinsically connected to all cellular processes that rely on the microtubule and actin filament components of the cytoskeleton, both of which are highly regulated and dynamic assemblies. In addition to CCT acting as a protein folding oligomer, further modes of CCT action mediated either by the CCT oligomer itself or via CCT subunits in their monomeric forms can influence processes associated with assembled actin filaments and microtubules. Thus, there is an extended functional role for CCT with regard to its major folding substrates with a complex interplay between CCT as folding machine for tubulin/actin and as a modulator of processes involving the assembled cytoskeleton. As cell division, directed cell migration, and invasion are major drivers of cancer development and rely on the microtubule and actin filament components of the cytoskeleton, CCT activity is fundamentally linked to cancer. Furthermore, the CCT oligomer also folds proteins connected to cell cycle progression and interacts with several other proteins that are linked to cancer such as tumor-suppressor proteins and regulators of the cytoskeleton, while CCT monomer function can influence cell migration. Thus, understanding CCT activity is important for many aspects of cancer cell biology and may reveal new ways to target tumor growth and invasion.

Keywords: Actin; Cancer; Chaperonin; Cytoskeleton; Tubulin.

Fig. 1

Structure of the CCT oligomer. a A three-dimensional reconstruction of the CCT oligomer following cryo-electron microscopy (Llorca et al. 2001). b Domain structure of CCT based on the structure of the thermosome (PDB: 1A6D) with the equatorial domain (red), the flexible linker (white), and the apical, substrate-binding domain (green) indicated. c The order of the CCT subunits within one chaperonin ring (Kalisman et al. ; Leitner et al. 2012). d A three-dimensional reconstruction of CCT-actin complexes (Llorca et al. 1999). e The signature residues of CCTγ identified by Pappenberger et al. (2002) mapped onto the structure of the apical domain of CCTγ (PDB: 1GN1). Hydrophobic residues are shown in red, others in green. f The putative substrate-binding site of CCTγ identified by Joachimiak et al. (2014) mapped onto the structure of the CCTγ apical domain. Hydrophobic residues are shown in red, others in green

Fig. 2

Model of the CCT folding cycle. a Actin (green) binds to the CCT oligomer in a 1.4 orientation. b ATP binding to high-affinity CCT subunits (red) leads to a powerstroke (black arrows) (Reissmann et al. 2012) and the actin molecule being released from one side of the chaperonin ring (Llorca et al. 2001). c After all CCT subunits have bound ATP, a sequential wave of ATP hydrolysis occurs either starting at CCTζ and proceeding clockwise around the ring (solid blue arrow, most probable) or starting at CCTθ and proceeding anti-clockwise around the ring (open blue arrow, less probable) (Gruber et al. 2017). Such a wave of ATP hydrolysis could be coupled to the ordered release of the folding substrate

Fig. 3

The complex interplay between CCT and actin and tubulin. Cartoon of a eukaryotic cell depicting the interactions between CCT and the actin- and tubulin-based cytoskeletal systems. Tubulin and actin folding. The CCT oligomer folds newly synthesized tubulin and actin (Sternlicht et al. 1993). Regulation of actin transcription. The CCTε subunit when monomeric can act as a component of the SRF pathway by interacting with the co-transcriptional activator MRTF-A (M) and thus has the potential to connect the folding capacity of the cell for actin to the transcription of actin (Elliott et al. 2015). MRTF-A is shown in the nucleus binding together with SRF to DNA sequences containing a CARG motif to initiate the transcription of the SRF genes that include actin and several actin-binding proteins (Sun et al. ; Vartiainen et al. 2007). Association with actin filaments. CCTε can associate with actin filament bundles and its levels as a monomer are linked to cell shape (Brackley and Grantham 2010). The CCT oligomer can affect the initial rate of actin polymerization but not the final levels of actin filaments in vitro (Grantham et al. 2002). The actin filament severing and capping protein gelsolin, in its Ca²⁺-bound conformation, binds to the CCT oligomer (Svanstrom and Grantham 2016) but is not a folding substrate of CCT (Brackley and Grantham 2011). Association with microtubules. Some CCT subunits behave as microtubule-associated proteins in vitro (Roobol et al. 1999). CCTδ monomer interacts with p150^Glued (a component of the dynactin complex linking the dynein motor to microtubules) in close proximity to the plasma membrane (Echbarthi et al. 2018)