In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.

Protein folding in the cell.

The folding of many newly synthesized proteins in the cell depends on a set of conserved proteins known as molecular chaperones. These prevent the formation of misfolded protein structures, both under normal conditions and when cells are exposed to stresses such as high temperature. Significant progress has been made in the understanding of the ATP-dependent mechanisms used by the Hsp70 and chaperonin families of molecular chaperones, which can cooperate to assist in folding new polypeptide chains.

Molecular chaperones in cellular protein folding.

Intrinsically unstructured proteins: re-assessing the protein structure-function paradigm.

The Hsp70 and Hsp60 chaperone machines.

T URNOVER OF AB ERR ANT PROT EINS IN E. COLI . . . . . . . . . . . . . . . . . 445 The Lon Protease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 DnaK. DnaJ. GrpE. GroEL and GroES . . . . . . . . . . . . . . . . . . . . . . . . . . 447 The Clp Protease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448 The DegP Protease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

The Function of Heat-Shock Proteins in Stress Tolerance: Degradation and Reactivation of Damaged Proteins

A nuclear encoded mitochondrial heat-shock protein hsp60 is required for the assembly into oligomeric complexes of proteins imported into the mitochondrial matrix. hsp60 is a member of the 'chaperonin' class of protein factors, which include the Escherichia coli groEL protein and the Rubisco subunit-binding protein of chloroplasts.

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Mitochondrial heat-shock protein hsp60 is essential for assembly of proteins imported into yeast mitochondria.

Folding of two monomeric enzymes mediated by groE has been reconstituted in vitro. The groEL protein stabilizes the polypeptides in a conformation resembling the 'molten globule' state. Mg-ATP and groES then promote the acquisition of ordered tertiary structure at the surface of groEL. Folding requires the hydrolysis of about 100 ATP molecules per protein monomer. This active process of surface-mediated chain folding might represent a general mechanism for the formation of protein structure in vivo.

Chaperonin-mediated protein folding at the surface of groEL through a 'molten globule'-like intermediate

Role of an energized inner membrane in mitochondrial protein import. Delta psi drives the movement of presequences.

THE MEMBERS OFTHE HSP70 FAMILY •• 298 Unfolding Is Required for Membrane Translocation of Proteins 299 Cytoso/ic Hsp70 Stabilizes Precursor Proteins for Translocation 300 Organellar Hsp70 in Protein Translocation and Folding 301 Requirement of Hsp70 for Protein Assembly in the ER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 Molecular Mechanism of Hsp70 Action .... .. . . . . . . . . . .. . .. . . .. ... . . . ........ 303 THE MEMBERS OF THE HSP60 FAMILY ... ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 306 Multiple Functions of Mitochondrial Hsp60 308 Molecular Mechanism of Chaperon in Action . .. . . . 311

/pdf/protein-folding-in-the-cell-the-role-of-molecular-chaperones-tzfk0n6xpz.pdf

Protein Folding in the Cell: The Role of Molecular Chaperones Hsp70 and Hsp60

Two distinct pathways of sorting and assembly of nuclear-encoded mitochondrial inner membrane proteins are described. In the first pathway, precursor proteins that carry amino-terminal targeting signals are initially translocated via contact sites between both mitochondrial membranes into the mitochondrial matrix. They become proteolytically processed, interact with the 60-kDa heat-shock protein hsp60 in the matrix and are retranslocated to the inner membrane. The sorting of subunit 9 of Neurospora crassa Fo-ATPase has been studied as an example. Fo subunit 9 belongs to that class of nuclear-encoded mitochondrial proteins which are evolutionarily derived from a prokaryotic ancestor according to the endosymbiont hypothesis. We suggest that after import into mitochondria, these proteins follow the ancestral sorting and assembly pathways established in prokaryotes (conservative sorting). On the other hand, ADP/ATP carrier was found not to require interaction with hsp60 for import and assembly. This agrees with previous findings that the ADP/ATP carrier possesses non-amino-terminal targeting signals and uses a different import receptor to other mitochondrial precursor proteins. It is proposed that the ADP/ATP carrier represents a class of mitochondrial inner membrane proteins which do not have a prokaryotic equivalent and thus appear to follow a non-conservative sorting pathway.

https://epub.ub.uni-muenchen.de/7547/1/Neupert_Walter_7547.pdf

Jörg Martin

Papers

Mitochondrial heat-shock protein hsp60 is essential for assembly of proteins imported into yeast mitochondria.

Chaperonin-mediated protein folding at the surface of groEL through a 'molten globule'-like intermediate

Role of an energized inner membrane in mitochondrial protein import. Delta psi drives the movement of presequences.

Protein Folding in the Cell: The Role of Molecular Chaperones Hsp70 and Hsp60

Sorting pathways of mitochondrial inner membrane proteins