Chaperonin 60: a paradoxical, evolutionarily conserved protein family with multiple moonlighting functions

Abstract

Chaperonin 60 is the prototypic molecular chaperone, an essential protein in eukaryotes and prokaryotes, whose sequence conservation provides an excellent basis for phylogenetic analysis. Escherichia coli chaperonin 60 (GroEL), the prototype of this family of proteins, has an established oligomeric-structure-based folding mechanism and a defined population of folding partners. However, there is a growing number of examples of chaperonin 60 proteins whose crystal structures and oligomeric composition are at variance with GroEL, suggesting that additional complexities in the protein-folding function of this protein should be expected. In addition, many organisms have multiple chaperonin 60 proteins, some of which have lost their protein-folding ability. It is emerging that this highly conserved protein has evolved a bewildering variety of additional biological functions - known as moonlighting functions - both within the cell and in the extracellular milieu. Indeed, in some organisms, it is these moonlighting functions that have been left after the loss of the protein-folding activity. This highlights the major paradox in the biology of chaperonin 60. This article reviews the relationship between the folding and non-folding (moonlighting) activities of the chaperonin 60 family and discusses current knowledge on their molecular evolution focusing on protein domains involved in the non-folding chaperonin functions in an attempt to understand the emerging biology of this evolutionarily ancient protein family.

Multiple chaperonins in bacteria—novel functions and non-canonical behaviors

“…As per the convention, groEL gene that is in operonic arrangement with groES has been termed groEL1, while that that exists separately on the genome has been termed groEL2 (Kong et al 1993;Cole et al 1998). Despite high sequence similarity to other chaperonins, functional studies on GroEL2 initially identified it as the immunodominant antigen in the mycobacterial culture filtrates, leading to the search for its immunological roles (Lamb et al 1989;Cehovin et al 2010;Henderson et al 2010Henderson et al , 2013. Therefore, studies that followed focused on understanding the immunological tasks of the Mtb GroELs, which eventually demonstrated that all the chaperonins, GroEL1 (Lewthwaite et al 2001), GroEL2 (Hickey et al 2010), and GroES (Sonnenberg and Belisle 1997), are secreted, with GroEL1 being the more immunopotent (Lewthwaite et al 2001).…”

Chaperonin-enhanced Escherichia coli cell-free expression of functional CXCR4

“…Meanwhile, based on essentially the same basic mechanism involving hydrophobic interaction, the bacterial GroEL-GroES is evolutionarily conserved to regulate many aspects of protein bioprocess in vivo, such as the well-recognized role of driving protein folding, maintaining proteostasis, and membrane protein solubilization, translocation and targeting (Castanie-Cornet et al, 2014;Henderson et al, 2013;Lin et al, 2006). Here, the ability of this remarkable molecular machine to modulate functional cell-free expression of CXCR4 has been examined.…”

The complexity of chloroplast chaperonins

“…(iii) Some subunits are defined by unique functions, unrelated to protein folding, which would preclude the ability to substitute for each other. Such 'moonlighting' roles have been observed for bacterial as well as mammalian chaperonins and have been discussed extensively in a recent review [31].…”

“…Heat-shock (p rotecƟon of Rubi sco acƟvase) [44] NdhH folding, assembly of the NDH complex [38] RegulaƟon via calmodulin bindin g [50] Chlo ropl ast di vision (FtsZ ring formaƟon) [40] TRENDS in Plant Science of chaperonin genes in various bacterial species [31,75]. (i) All of the proteins have a similar role, in which case the genes should all be interchangeable, but are differentially expressed, enabling regulation under different conditions.…”

“…Many bacterial species also contain several Cpn60 and/or Cpn10 homologs, and substantial progress has been made in understanding the structural basis for different functions of multiple bacterial chaperonins, however, this topic has been extensively reviewed elsewhere [31,32]. Similar structure-function studies of chloroplast chaperonins, with their intricate compositions, are only in their infancy.…”