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Chapter 10 : Protein-Folding Systems
Category: Microbial Genetics and Molecular Biology; Environmental Microbiology
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This chapter describes the known members of archaeal protein-folding pathways, including not only the heat-shock-regulated members, but also the non-heat-shock-regulated protein chaperones. The major chaperone classes, heat shock protein (Hsp) 100 and Hsp90/Hsp83, are absent from the genomes of the hyperthermophilic archaea, although they are present in several mesophilic and thermophilic archaea. In archaea, with one exception, prefoldins are hexamers consisting of two α-subunits and four β-subunits, which act as generalized holding chaperones. The holding-and-release mechanism of the archaeal prefoldins has recently been elucidated. The archaeal group II chaperonins form toroidal double rings with an eightor ninefold symmetry, consisting of homologous subunits. The subunit composition of the chaperonin complexes in several archaea changes with growth temperature. The known properties, arrest and ATPase activity, and structural characteristics of archaeal chaperonins are provided in this chapter. The helical protrusion is strictly conserved among group II chaperonins. The existing evidence indicates that asymmetric and symmetric molecules are present in the functional ATPase cycle of archaeal group II chaperonins. The coexistence of both groups of chaperonins in the same cytosol in the Methanosarcina species provides a useful model system for studying the differential substrate specificities of the group I and II chaperonins, and for elucidating how newly synthesized proteins are sorted from the ribosome to the appropriate chaperonin for folding.
Structure of the prefoldin from M. thermautotrophicus. The globular body and coiled coils extensions in the “jellyfish” model form an adjustable cage that accommodates and binds proteins by a clamp mechanism (59). Reproduced from Nature Reviews Microbiology (53) with permission of the publisher.
Effect of chaperones sHsp and Hsp60 on the thermostability of Taq DNA polymerase in the presence of P. furiosus molecular chaperones. (A) Inactivation of Taq polymerase in the presence of individual subunits of sHsp (Δ), Hsp60 (□), Hsp60-Mg2+-ATP (▪), sHsp and HSP60 (◊), and sHsp and Hsp60-Mg2+-ATP (♦). The controls are reactions without the addition of chaperones (◦) and with the addition of Mg2+ and ATP (•). (B) Inactivation of Taq polymerase in the presence of individual subunits of prefoldin, prefoldin α (Δ) and β (▲), prefoldin complex (?), Hsp60 (□), Hsp60-Mg2+-ATP (▪), prefoldin and HSP60 (◊), and prefoldin and Hsp60-Mg2+-ATP (♦). The controls are reactions without the addition of chaperones (◦) and with the addition of Mg2+ and ATP (•). Reproduced from Biotechnology and Bioengineering (52) with permission of the publisher.
Schematic model for the reaction mechanism of archaeal group II chaperonins. See text for details. A, I, and E refer to the apical, intermediate, and equatorial domains, respectively. H represents the helical protrusion. Reproduced from Journal of Biological Chemistry (104) with permission of the publisher.
Occurrence of different classes of HSPs in the three domains
Archaeal chaperonin: number of subunits encoded per genome
Structural and functional characteristics of archaeal group II chaperonins