HSP60: Species Variation

IHC staining of Listeria infected mice spleens 6 days after infection with L-monocytogenes, using Anti-Hsp60 (clone: LK2)

The 60 kDa chaperonin family has homologs in many eukaryotes like Danio rerio, D. melanogaster, yeast, and plants as well as many eubacteria and Archaea. Chaperonins represent a diverse family of molecular chaperones that are present not only in mitochondria as well as cytoplasm of all eukaryotes and eubacteria but also in plastids. Plastidic Cpn60 appears to be distinct from homologous proteins in bacteria and mitochondria in that it is composed of two distinct subunit types, α and β 93, 94. An examination of the Arabidopsis thaliana genome sequence led to the identification of 29 predicted genes with the potential to encode members of the chaperonin family of chaperones (Hsp60, Cpn60) 95. Amongst them, eleven Arabidopsis genomic sequences were identified as having potential to encode Cpn60 proteins. Six of these appeared to encode plastidic Cpn60 subunits, including 2 α-subunits and 4 β-subunits (Table 2). Each harbors a putative transit peptide sequence. A seventh genomic sequence appeared to represent a plastidic Cpn60 β-subunit pseudogene. The remaining four genomic sequences comprise three genes encoding mitochondrial Cpn60 polypeptides (Table 2) and an apparent mitochondrial protein pseudogene 95. Hsp60-2 is the longest isoform with 585 amino acids, while Hsp60-3B is composed of 572 amino acids and Hsp60 is composed of 577 residues, respectively. The primary structures of plastidic α- and β-subunits and mitochondrial subunits show a high grade of divergence. While α- and β-subunits of A. thaliana Cpn60 have an average 51% identity to each other, they show an approximately 45% identity to the mitochondrial Cpn60 protein. These intersubunit identities are comparable to those found between prokaryotic Cpn60 homologs and any of the eukaryotic subunits 95. The Cpn60A paralogs are approximately 57% identical in peptide sequence whereas three Cpn60B paralogs (Cpn60B1 to -3) show an average 88% identity. In contrast, the fourth paralog, Cpn60-B4 is only 60% identical to each of the other three paralogs 95. Genes encoding the distinct plastidic subunit types obviously originated from a gene duplication event occurring in the plastid lineages 84, 96, 97. Therefore, α- and β-genes can be considered as being paralogs. It is worth mentioning that genes encoding plastidic and mitochondrial homologs derived from independent endosymbiotic events.

The thorough search of the genome sequence of A. thaliana also revealed nine predicted coding regions distributed among chromosomes 1, 3, and 5 encoding the CCT proteins CCT-α, -β, -γ, -δ, -ε, -η, – θ, -ξ in which CCT-ξ is encoded by two regions 95. A similar arrangement exists not only in humans but also in mice where a second CCT-ξ subunit is expressed in testes 60, 86.

The genome of the unicellular green alga Chlamydomonas reinhardtii encodes four Hsp60 chaperones, termed Cpn60A, -B1, -B2, and -C. Cpn60A as well as Cpn60-B1/-2 are targeted to chloroplasts, while Cpn60C is targeted to mitochondria. CPN60Bs from higher plants may form homo- or hetero-oligomeric stacks of two heptameric rings in conjunction with Cpn60A. Similar to higher plants, C. reinhardtii harbors divergent plastidic Cpn60A and Cpn60B subunits 98. Cpn60A shares only ≈50% similarity with chloroplast-targeted CPN60Bs wherease Cpn60-B1 shows a 79% identity to Cpn60-B2 and vice versa. Cpn60C is targeted to mitochondria where it forms stacks of two homo-oligomeric heptameric rings. For complete functionality, Cpn60C requires mitochondrial Cpn10 in an ATP-dependent manner. Since the primary structures of the two Chlamydomonas β-subunits only show a ≈25% divergence, it can be hypothesized that the two Cpn60-B subunits have been selectively maintained for a considerable period of time 98. It is interesting to note that, unlike plastidic chaperonin transcripts in higher plants, plastidic Cpn60 mRNA levels from C. reinhardtii rapidly increase in response to heat shock 98.

Members of the HSP60 family of chaperones are generally not induced by heat shock in Drosophila tissues 13. except in Malpighian tubules 99. The Berkeley Drosophila Genome Project has reveled four Hsp60 genes, named as Hsp60A, Hsp60B, Hsp60C, and Hsp60D, respectively 100. Previous studies have shown that the Hsp60A, Hsp60B, and Hsp60C proteins have distinct functions in normal development. The Hsp60A gene is expressed in all cell types of Drosophila and is essential from early embryonic stages 101, while the Hsp60B gene is expressed only in testis, being essential for sperm individualization 102. Studies by the group of Surajit Sakar and Subash Lakhotia confirmed the crucial role of Hsp60C in proper tracheal development and in early stages of spermatogenesis 100 as well as in oogenesis, especially in organization and maintenance of cytoskeletal and cell adhesion components 103. Hsp60D encodes a protein harboring sequence homology with bacterial GroEL and human HspD1. Transcripts of this gene are ubiquitously found in larval tissues such as imaginal discs and salivary glands 104. Recent data suggest an essential contribution of Hsp60D to caspase-mediated apoptosis in D. melanogaster through its interaction with Drosophila inhibitor of apoptosis 1 (DIAP-1) 104, 105.

The GroEL and GroES proteins and regulation of their expression have been studied in most detail in E. coli. The groES and groEL genes form the groESL (GroE) operon essential for E. coli viability at any temperature 106. The groESL operon of E. coli and other bacteria analyzed are arranged in the order promoter–groES–groEL 107. Several bacteria harbor a further, monocistronic groEL operon including Synechocystis sp. 108, Synechococcus vulcanus 109, Rhizobium meliloti 110, and the cyanobacterium Anabaena sp. 111.

Many bacteria have multiple copies of the groEL gene which are active under different environmental conditions consequently leading to the expression of various Hsp60 homologs. This includes a number of Gram-positive bacteria such as M. leprae, S. albus, M. tuberculosis and cyanobacteria 108, 112, 113, 114 (see Table 2) as well as several Gram-negative bacteria also expressing multiple Hsp60 homologs 110, 115, 116. Unlike all other prokaryotes, the nitrogen fixing soybean root nodule bacterium, Bradyrhizobium diazoefficiens (old name B. japonicum), possesses a multigene family consisting of seven very similar groEL-like genes 64. These homologs are expressed to different degrees by differentially regulated groESL operons. Amongst them, one family member (GroEL-3) is induced by a mechanism that does not involve the well-known heat shock response 116. Interestingly, its synthesis is co-regulated at the transcriptional level with the process of symbiotic nitrogen fixation via the oxygen-responsive transcriptional activator NifA and the σ-factor RpoN (afM4).

Unlike the homo-oligomer GroEL, archaeal chaperonins are often composed of several distinct, but homologous subunits. The archaeal chaperonins belong to the group II chaperonins (called the thermosome, TF55 or CCT complex – analogous to the eukaryotic molecules), and are strongly induced by heat shock 61. Archaea posses one to three homologous chaperonin-encoding genes (see also Table 2) except for Methanosarcina acetivorans which expresses five genes 65. Phylogenetic analyses imply that multiple independent duplications in archaeal chaperonin genes occurred in archaeal lineages facilitating the presence of multiple chaperonin genes 97, 117. In archaeal genomes, duplicate chaperonin genes (paralogs) are often more similar to each other than to those in other archaea, suggesting recent (lineage-specific) duplications 84. As stated by Archibald and collaborators, the persistence of chaperonin paralogs in multiple archaeal lineages may involve a process of co-evolution, where chaperonin heterogeneity is altered independently of selection on function 117. Although archaeal chaperonins are strongly induced by heat in vitro 118, their in vivo function remains unclear. In a recent study, the group of Igor K. Lendnev observed that Hsp60 from hyperthermophile Pyrococcus furiosus binds to and degrades insulin fibrils. Prolonged incubation with the chaperonin was found to produce large amorphous aggregates with polydisperse topologies 119. Based on these findings the authors postulate a novel approach to the disassembly of refractory protein aggregates under physiological conditions. However, a contribution of archaeal chaperonins to ribosomal RNA processing is also quite likely 120, 121.

Compared to archaeal chaperonins, the eukaryotic CCT chaperonin complex TRiC is even more hetero-oligomeric 52, 122. Previous comparative sequence analyses identified this complex in the common ancestor of animals and fungi 83. The results by Archibald and colleagues ascribe the origin of the CCT gene duplications to the common ancestor of animals, fungi, plants, parabasalids, and diplomonads, and likely to the common ancestor of all extant eukaryotes 84. Sequence comparisons of CCT genes in mouse, encoding the subunits of TRiC, confirmed the existence of eight distinct subunit species (α, β, γ, δ, ϵ, η, θ, and ζ), each thought to occupy a unique position in the octameric CCT rings 52, 83, 85, 123, 124. As suggested by the group of W. Ford Doolittle 84, the divergent nature of these genes, together with the discovery of clear yeast orthologs (see Table 2) to each of the mouse subunits 82, 83, 125, might imply an ancient paralogy within eukaryotes. Apart from the classical TRiC-interacting cytoskeletal proteins actin and tubulin, a genome-wide analysis of chaperonin function in yeast identified a third connection between TRiC and the cytoskeleton, the septin ring whose assembly and function has been shown to clearly depend on the interaction with TRiC 126. S. cerevisiae also expresses the mitochondrial Hsp60 which exhibits striking amino acid sequence similarity to its counterparts in humans, plants, and bacteria indicating a high degree of conservation 127. The yeast Hsp60 protein is encoded by the HSP60 gene (also known as MIF4, CPN60 or MNA2) in which mutations give rise to phenotypes characterized by mitochondrial dysfunction 128.

Higher eukaryotes such as C. elegans and zebrafish (D. rerio) also express multiple HSP60 genes (Table 2). A genetic approach revealed an upregulated expression of hspd1 (nbl) in blastema cells during zebrafish fin regeneration 32. The study also highlighted the pivotal role of HspD1 in blastema formation and viability in D. rerio. Mutations in CCT-3 (Cct-γ) lead to the no tectal neuron phenotype in zebrafish, interfering with retinotectal development 129. A growing body of evidence now indicates that HspD1 is critically involved in gametogenesis of C. elegans 130, and in spermatogenesis of rats 131, 132 and men 133. In the latter, a significant reduction in HspD1-immunopositive spermatogonia could be observed in testes with maturation arrest of spermatogenesis at the level of primary spermatocytes compared with testes exhibiting normal spermatogenesis 133.


Table 2: HSP60s of various pro- and eukaryotic organisms

Gene Protein Aliases UniProt ID Gene ID
 HSPD1  HspD1 Hsp60, mitochondrial Hsp60 (mtHsp60), 60 kDa chaperonin (Cpn60), mitochondrial matrix protein P1, P60 lymphocyte protein P10809 3329
 CCT1  Cct-1 CCT-1, TCP-1, CCTA, CCT-alpha, T-complex protein 1 subunit alpha (TCP-1-alpha) P17987 6950
 CCT2  Cct-2 CCT-2, CCTB, CCT-beta, T-complex protein 1 subunit beta (TCP-1-beta) P78371 10576
 CCT3  Cct-3 CCT-3, CCTG, CCT-gamma, T-complex protein 1 subunit gamma (TCP-1-gamma), hTRiC5 P49368 7203
 CCT4  Cct-4 CCT-4, CCTD, CCT-delta, T-complex protein 1 subunit delta (TCP-1-delta), stimulator of TAR RNA-binding P50991 10575
 CCT5  Cct-5 CCT-5, CCTE, CCT-epsilon, T-complex protein 1 subunit epsilon (TCP-1-epsilon) P48643 22948
 CCT6A  Cct-6A CCT-6A, CCTZ, CCT-zeta, CCT-zeta-1, HTR3,
Tcp-20, T-complex protein 1 subunit zeta (TCP-1-zeta-1), acute morphine dependence-related protein 2
P40227 908
 CCT6B Cct-6B CCT-6B, CCTZ2, CCT-zeta-2, testis-specific protein TSA303, T-complex protein 1 subunit zeta-2 (TCP-1-zeta-2), Testis-specific Tcp-20 Q92526 10693
 CCT7  Cct-7 CCT-7, CCTH, CCT-eta, T-complex protein 1 subunit eta (TCP-1-eta), HIV-1 Nef-interacting protein Q99832 10574
 CCT8  Cct-8 CCT-8, CCTQ, CCT-theta, T-complex protein 1 subunit theta (TCP-1-theta), KIAA002, renal carcinoma antigen NY-REN-15 P50990 10694
D. rerio
 hspd1  HspD1 60 kDa heat shock protein, hsp60, no blastema (nbl), mitochondrial
Nbl, heat shock 60kDa protein 1 (chaperonin)
 Q803B0  282676
 cct1  Cct-1 CCT-1, TCP1, CCTA, CCT-alpha, T-complex protein 1 subunit alpha (TCP-1-alpha), T-complex polypeptide 1  Q9W792  30477
 cct2  Cct-2 CCT-2, CCTB, CCT-beta, T-complex protein 1 subunit beta (TCP-1-beta), TCP-1 subunit 2  Q6PBW6  192326
 cct3  Cct-3 CCT-3, CCTG, CCT-gamma, T-complex protein 1 subunit gamma (TCP-1-gamma), nTn, no tectal neuron, TCP-1 subunit 3  Q8JHI7  192327
 cct4  Cct-4 CCT-4, CCTD, CCT-delta, T-complex protein 1 subunit delta (TCP-1-delta)  Q6PH46  393555
 cct5  Cct-5 CCT-5, CCTE, CCT-epsilon, T-complex protein 1 subunit epsilon (TCP-1-epsilon)  Q6NVI6  322258
 cct6a  Cct-6A CCT-6A, CCTZ, CCT-zeta, T-complex protein 1 subunit zeta (TCP-1-zeta)  Q7ZYX4  116994
 cct7  Cct-7 CCT-7, CCTH, CCT-eta, T-complex protein 1 subunit eta (TCP-1-eta), TCP-1 subunit 7, etID46398.7, etID46398.12  Q8JHG7  192324
 cct8  Cct-8 CCT-8, CCTQ, CCT-theta, T-complex protein 1 subunit theta (TCP-1-theta), bette davis (bdav)  Q7ZU96  Q7ZU96
D. melanogaster
 Hsp60A  Hsp60A Hsp60, HSP-60, HSP60A, hsp60A, 60-kDa heat shock protein, mitochondrial; CG12101-PA, CG12101-PB, Hsp60-PA, Hsp60-PB, lethal(1)10Ac, mitochondrial matrix protein P1, 60 kDa chaperonin (Cpn60), Dmhsp60, Dmel\CG12101  O02649  32045
 Hsp60B  Hsp60B Hsp60b, Dmel\CG2830, CG2830-PA, Hsp60 related, Hsp60B-PA, male sterile 2(21)D, 60 kDa chaperonin (Cpn60); 60 kDa heat shock protein homolog 1, mitochondrial  Q9VPS5  48572
 Hsp60C  Hsp60C Dmel\CG7235, CG7235-PA, CG7235-PB, CG7235-PC, Hsp60C-PA, Hsp60C-PB, Hsp60C-PC, 60 kDa chaperonin (Cpn60); 60 kDa heat shock protein homolog 2, mitochondrial  Q9VMN5  33796
 Hsp60D  Hsp60D Dmel\CG16954, CG16954-PA, CG16954-PB, Hsp60D-PA, Hsp60D-PB, AT04835p  Q9VJX7  34763
S. cerevisiae
 HSP60  Hsp60 Hsp60, mitochondrial Hsp60 (mtHsp60), 60 kDa chaperonin (Cpn60), P66, stimulator factor I 66 kDa component  P19882  850963
 CCT1  Cct-1 CCT-1, TCP-1, CCTA, CCT-alpha, T-complex protein 1 subunit alpha (TCP-1-alpha)  P12612  851798
 CCT2  Cct-2 CCT-2, CCT-beta, TCP-2, T-complex protein 1 subunit beta (TCP-1-beta)  P39076  854664
 CCT3  Cct-3 CCT-3, CCT-gamma, TCP-3, T-complex protein1 subunit gamma (TCP-1-gamma)  P39077  853438
 CCT4  Cct-4 CCT-4, CCT-delta, TCP-4, T-complex protein1 subunit delta (TCP-1-delta)  P39078  851412
 CCT5  Cct-5 CCT-5, TCP-5, T-complex protein1 subunit epsilon (TCP-1-epsilon)  P40413  853527
 CCT6  Cct-6 CCT-6, TCP-6, T-complex protein1 subunit zeta (TCP-1-zeta), Tcp-20, HTR3  P39079  851768
 CCT7  Cct-7 CCT-7, TCP-7, T-complex protein1 subunit eta (TCP-1-eta)  P42943  853333
 CCT8  Cct-8 CCT-8, T-complex protein1 subunit theta (TCP-1-theta)  P47079  853447
A. thaliana
 HSP60  Hsp60 Chaperonin Cpn60, mitochondrial; HSP60-3B, 60 kDa chaperonin (Cpn60)  P29197  821983
 HSP60-2  Hsp60-2 Chaperonin Cpn60-like 1, mitochondrial; Hsp60-like 1  Q8L7B5  817883
 HSP60-3A  Hsp60-3A Chaperonin Cpn60-like 2, mitochondrial; Hsp60-like 2, Hsp60-3A  Q93ZM7  820599
 CPN60A1  Cpn60-A1 Chloroplastic chaperonin 60 subunit alpha 1, schlepperless, SLP, RuBisCO large subunit-binding protein subunit alpha 1, T1E2.8, T1E2_8, Cpn80-alpha 1  P21238  817344
 Cpn60-A2 Chloroplastic chaperonin 60 subunit alpha 2, Cpn60-alpha 2, EMBRYO DEFECTIVE 3007  Q56XV8  832000
 CPN60B1  Cpn60-B1 Chloroplastic chaperonin 60 subunit beta 1; RuBisCO large subunit-binding protein subunit beta, chloroplastic; Cpn60-beta 1  P21240  841996
 CPN60B2  Cpn60-B2 Chloroplastic chaperonin 60 subunit beta 2, Cpn60-beta 2  Q9LJE4  820549
 Cpn60-B3 Chloroplastic chaperonin 60 subunit beta 3, Cpn60-beta 3  C0Z361  835751
 Cpn60-B4 Chloroplastic chaperonin 60 subunit beta 4, Cpn60-beta 4  Q9C667  839164
C. reinhardtii
 CPN60A  Cpn60A Chaperonin 60A (GroEL/HSP60-homolog), Cpn60-alpha  A8JIB7  5729243
 CPN60B1  Cpn60-B1 Chaperonin 60B1 (GroEL/HSP60-homolog), Cpn60-beta1  A8JE91  5726659
 CPN60B2  Cpn60-B2 Chaperonin 60B2 (GroEL/HSP60-homolog),  A8ITH8  5717970
 CPN60C  Cpn60C Chaperonin 60C (GroEL/HSP60-homolog)  A8IMK1  5716829
E. coli
 groEL (groL)  GroEL 60 kDa chaperonin, Cpn60  B1XDP7  6061450
M. leprae
 groEL1 (groL1)  GroEL-1 60 kDa chaperonin 1, Cpn60-1  P37578  909064
 groEL2 (groL2)  GroEL-2 60 kDa chaperonin 2, Cpn60-2, 65 kDa antigen, Hsp65  P09239 908906
B. diazoefficiens
 groEL1 (groL1)  GroEL-1 60 kDa chaperonin 1, Cpn60-1  P77829  1051462
 groEL2 (groL2)  GroEL-2 60 kDa chaperonin 2, Cpn60-2  P35861  1048307
 groEL3 (groL3)  GroEL-3 60 kDa chaperonin 3, Cpn60-3  P35862  1055489
 groEL4 (groL4)  GroEL-4 60 kDa chaperonin 4, Cpn60-4  Q89P00  1051467
 groEL5 (groL5)  GroEL-5 60 kDa chaperonin 5, Cpn60-5  Q89LB1  1052432
 groEL6 (groL6)  GroEL-6 60 kDa chaperonin 6, Cpn60-6  Q89IK8  1053427
 groEL7 (groL7)  GroEL-7 60 kDa chaperonin 7, Cpn60-7  Q89DA6  1047245
Pyrococcus furiosus
 PF1974  Thermosome TF55 (thermophilic factor 55), CCT, Hsp60  Q8TZL6  1469856