[The interaction of the GroEL chaperone with early kinetic intermediates of renaturing proteins inhibits the formation of their native structure]. / Vzaimodeistvie shaperona GroEL s rannimi kineticheskimi promezhutochnymi sostoianiiami renaturiruiushchikh belkov ingibiruet formirovanie ikh nativnoi struktury.

The main function of the chaperone GroEL is to prevent nonspecific association of nonnative protein chains and provide their correct folding. In the present work, the renaturation kinetics of three globular proteins (human alpha-lactalbumin, bovine carbonic anhydrase, and yeast phosphoglycerate kinase) in the presence of different molar excess of GroEL (up to 10-fold) was studied. It was shown that the formation of the native structure during the refolding of these proteins is retarded with an increase in GroEL molar excess due to the interaction of kinetic protein intermediates with the chaperone. Mg(2+)-ATP and Mg(2+)-ADP weaken this interaction and decrease the retarding effect of GroEL on the protein refolding kinetics. The theoretical modeling of protein folding in the presence of GroEL showed that the experimentally observed linear increase in the protein refolding half-time with increasing molar excess of GroEL must occur only when the protein adopts its native structure outside of GroEL (i.e. in the free state), while the refolding of the protein in the complex with GroEL is inhibited. The dissociation constants of GroEL complexed with the kinetic intermediates of the proteins studied were evaluated, and a simple mechanism of the functioning of GroEL as a molecular chaperone was proposed.

Evidence for coupling of membrane targeting and function of the signal recognition particle (SRP) receptor FtsY.

Recent studies have indicated that FtsY, the signal recognition particle receptor of Escherichia coli, plays a central role in membrane protein biogenesis. For proper function, FtsY must be targeted to the membrane, but its membrane-targeting pathway is unknown. We investigated the relationship between targeting and function of FtsY in vivo, by separating its catalytic domain (NG) from its putative targeting domain (A) by three means: expression of split ftsY, insertion of various spacers between A and NG, and separation of A and NG by in vivo proteolysis. Proteolytic separation of A and NG does not abolish function, whereas separation by long linkers or expression of split ftsY is detrimental. We propose that proteolytic cleavage of FtsY occurs after completion of co-translational targeting and assembly of NG. In contrast, separation by other means may interrupt proper synchronization of co-translational targeting and membrane assembly of NG. The co-translational interaction of FtsY with the membrane was confirmed by in vitro experiments.

New prospects in studying the bacterial signal recognition particle pathway.

In vivo and in vitro studies have suggested that the bacterial version of the mammalian signal recognition particle (SRP) system plays an essential and selective role in protein biogenesis. The bacterial SRP system consists of at least two proteins and an RNA molecule (termed Ffh, FtsY and 4.5S RNA, respectively, in Escherichia coli). Recent evidence suggests that other putative bacterial-specific SRP components may also exist. In vitro experiments confirmed the expected basic features of the bacterial SRP system by demonstrating interactions among the SRP components themselves, between them and ribosomes, ribosome-linked hydrophobic nascent polypeptides or inner membranes. The availability of a conserved (and essential) bacterial SRP version has facilitated the implementation of powerful genetic and biochemical approaches for studying the cascade of events during the SRP-mediated targeting process in vivo and in vitro as well as the three-dimensional structures and the properties of each SRP component and complex.

Targeting of GroEL to SecA on the cytoplasmic membrane of Escherichia coli.

Chaperonin GroEL has been found to interact with isolated cytoplasmic membrane of Escherichia coli. Interaction requires Mg ions, whereas MgATP inhibits, and inhibition is stronger in the presence of co-chaperonin GroES. "Heat-shock" of the membrane at 45 degrees C destroys irreversibly its ability to bind GroEL. The binding of GroEL is characterized by saturation with a maximum of about 100 pmol GroEL bound per mg of total membrane protein, indicating a limited capacity and specificity of the membrane to bind GroEL. According to results of immunoblotting analysis and cleavable photoactivable cross-linking, a membrane target of GroEL is SecA, a protein known as a central component of the translocation machinery. Moreover, in some cases GroEL could modulate a cycle of association of SecA with the membrane by stimulating release of SecA from the membrane. A physiological role of targeting of GroEL in or close to the protein-conducting membrane apparatus is discussed.

On the distribution of ligands within the asymmetric chaperonin complex, GroEL14.ADP7.GroES7.

In the presence of MgATP or MgADP the E. coli chaperonin proteins, GroEL and GroES, form a stable asymmetric complex with a stoichiometry of two GroEL7:one GroES7: seven MgADP. The distribution of the ligands between the two heptameric GroEL rings is crucial to our understanding of the mechanism of chaperonin-assisted folding, being either cis (i.e. [GroEL7.MgADP7.GroES7]-[GroEL7]) or trans (i.e. [GroEL7.MgADP7]-[GroEL7.GroES7]. On the basis of cross-linking experiments with 8-azido-ATP and the heterobifunctional reagent, N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP), it was suggested that GroES and MgADP are bound to the same GroEL ring which resists proteinase K digestion [Nature 366 (1993) 228-233]. However, we find that the SPDP-promoted cross linking of GroES and GroEL occurs in the absence of Mg2+, ADP or ATP, which are required for the formation of the asymmetric complex. Cross-linking is shown to occur only when the SPDP-modified GroES is co-precipitated with GroEL by trichloracetic acid. Furthermore, there are structural grounds for questioning whether SPDP can crosslink, in a physiologically relevant manner, an amino group of GroES with any of the cysteinyl groups of GroEL.

ATP induces non-identity of two rings in chaperonin GroEL.

For its function, the Escherichia coli chaperonin GroEL requires the presence of ATP and co-chaperonin GroES. We have observed that ADP displays a two-step inhibition of GroEL-dependent ATP hydrolysis, wherein one-half of the GroEL ATPase sites is strongly inhibited by ADP while the other half is affected very mildly. It is suggested that interaction with ATP induces structural and functional differences between two initially identical rings in GroEL (inter-ring negative cooperativity) and that the subsequent binding of GroES occurs to the ring that is occupied first by ATP in a positively cooperative manner.

Chaperonins as potential gene regulatory factors. In vitro interaction and solubilization of NifA, the nif transcriptional activator, with GroEL.

A previous study (Govezensky, D., Greener, T., and Zamir, A. (1991) J. Bacteriol. 20, 6339-6346) indicated that the chaperonin GroEL was required for maximal expression from nif promoters in Klebsiella pneumoniae and nif-transformed Escherichia coli. That this requirement stemmed from the ability of GroEL to properly fold NifA, the nif transcriptional activator, was first supported by co-immunoprecipitation of NifA in K. pneumoniae extracts with anti-GroEL antibodies. In the present in vitro study, NifA, partially purified from E. coli overexpressing the protein, was diluted from a 6 M urea solution into a refolding buffer in the presence or absence of GroEL. Dilution in the absence of GroEL caused the complete precipitation of NifA. When present in the dilution buffer, GroEL bound NifA and maintained it in a soluble state. GroEL was also found to bind NifA newly synthesized in an in vitro translation system. For both NifA preparations, cochaperonin GroES and ATP promoted release of NifA from GroEL. These results provide evidence for the association of NifA with GroEL and for the role of both GroEL and GroES in the solubilization and thereby folding of the nif transcriptional activator.

Prediction of an inter-residue interaction in the chaperonin GroEL from multiple sequence alignment is confirmed by double-mutant cycle analysis.

A search for co-ordinated amino acid changes in the hsp60 family of chaperonins suggested that cysteine residues at positions 137 and 518 in the Escherichia coli chaperonin GroEL may interact with each other. In order to determine whether this interaction indeed exists we constructed a double-mutant cycle comprising wild-type GroEL, the single mutants Cys137-->Ser and Cys518-->Ser and the corresponding double mutant. The effects of the two mutations on the function of GroEL, in assisting the refolding of a non-folded protein substrate (rhodanese), are shown to be non-additive. It is also shown that ADP by itself specifically destabilizes the Cys518-->Ser mutant GroEL particle with this effect being suppressed in the double mutant. The observed pattern of co-ordinated mutations in the hsp60 family of chaperonins is thus shown to reflect a real interaction, though most likely indirect, between Cys137 and Cys518 in GroEL. Our study demonstrates that patterns of co-ordinated mutations combined with double-mutant cycle analysis can provide structural information on interactions in a protein without an available three-dimensional structure at atomic resolution.

Direct demonstration that ATP is in contact with Cys-137 in chaperonin GroEL.

The nonhydrolyzable ATP analogue ATP gamma S (adenosine 5'-3-O-(thio)triphosphate) is affinity cross-linked to GroEL by formation of a disulfide bridge in a peroxide-promoted reaction. By replacing with serine each of 3 cysteine residues in GroEL, it is shown that ATP gamma S specifically cross-links to Cys-137. It is thus demonstrated that the ATP bound to GroEL is in direct contact with Cys-137.

The N terminus of the molecular chaperonin GroEL is a crucial structural element for its assembly.

The Escherichia coli heat-shock protein GroEL is a member of the highly conserved family of tetradecameric chaperonins 60, which assist in the folding and assembly of other proteins. Using site-directed mutagenesis, it is shown that replacement of the absolutely conserved amino acid residue Lys-3 by arginine or isoleucine destabilizes the GroEL particle and that the replacement Lys-3-->Glu completely blocks its formation. The rank order of effects of these mutations on the stability of the GroEL particle correlates with the associated changes in net charge at that position. Our results show that the N terminus of GroEL is a crucial structural element for its assembly.

Mutation Ala2-->Ser destabilizes intersubunit interactions in the molecular chaperone GroEL.

The mutation Ala2-->Ser in the molecular chaperone GroEL increases positive co-operativity in ATP hydrolysis, as reflected by a change in the Hill coefficient from 2.36(+/- 0.23) for wild-type to 3.19(+/- 0.17) for the mutant. This amino acid replacement destabilizes the oligomeric structure of GroEL. It is shown that adenine nucleotides also have a specific destabilizing effect which is more pronounced in the case of the Ala2-->Ser mutant. Addition of GroES or the non-folded protein ligand rhodanese blocks the destabilizing effect of adenine nucleotides for both wild-type and mutant. The results are interpreted using the Monod-Wyman-Changeux (MWC) model for co-operativity.

A newly synthesized protein interacts with GroES on the surface of chaperonin GroEL.

To facilitate folding and assembly of different proteins, chaperonin GroEL requires the presence of its helper protein GroES. Using a photochemical cross-linking approach, we show that GroES and newly synthesized pre-beta-lactamase (pre-beta lac) contact with each other only within the ternary complex with GroEL. Possibly owing to this contact GroES is able to directly influence the pre-beta lac/GroEL interaction. Furthermore, the cross-linking of pre-beta lac to GroES suggests that the binding of the protein ligands to GroEL occurs near the GroES binding site, known to be in the central hole space of GroEL.

Positive cooperativity in the functioning of molecular chaperone GroEL.

In the presence of its partner, GroES, the tetradecameric molecular chaperone GroEL binds 14 ATP molecules, half of which are hydrolyzed in a cooperative manner. Moreover GroEL can bind, with a positive cooperativity, more than two molecules of nonfolded protein rhodanese. The role of the cooperative mechanism in the functioning of GroEL is discussed.

Transient association of newly synthesized unfolded proteins with the heat-shock GroEL protein.

It has been suggested that newly synthesized proteins are maintained in their unfolded state by cellular ATP-driven factors which may prevent or reverse the formation of misfolded structures or promote the correct assembly of oligomeric proteins or post-translational secretion. Using a photocross-linking approach, we have identified the 20S heat-shock GroEL protein as the major cytosolic component which forms a complex with the unfolded newly synthesized pre-beta-lactamase or chloramphenicol acetyltransferase in Escherichia coli. Dissociation of these complexes is ATP-dependent. The unfolded state of pre-beta-lactamase, maintained by the transient interaction with GroEL, may be essential for the secretion of this protein.

The signal sequence of nascent preprolactin interacts with the 54K polypeptide of the signal recognition particle.

Hydrophobic signal sequences direct the translocation of nascent secretory proteins and many membrane proteins across the membrane of the endoplasmic reticulum. Initiation of this process involves the signal recognition particle (SRP), which consists of six polypeptide chains and a 7S RNA and interacts with ribosomes carrying nascent secretory polypeptide chains. In the case of aminoterminal, cleavable signal sequences, in the absence of microsomal membranes it exerts a site-specific translational arrest in vitro. The size of the arrested fragment (60-70 amino-acid residues) suggests that elongation stops when the signal sequence has emerged fully from the ribosome. However, a direct interaction between the signal sequence and SRP has not previously been demonstrated and has even been questioned recently. We now show for the first time a direct interaction between the signal sequence of a secretory protein and a component of SRP, the 45K polypeptide (relative molecular mass (Mr) 54,000). This was achieved by means of a new method of affinity labelling which involves the translational incorporation of an amino acid, carrying a photoreactive group, into nascent polypeptides.

Localization of elongation factor Tu on the ribosome.

The EF-Tu-binding center of the E. coli ribosome has been localized by immunoelectron microscopy after cross-linking of the specific EF-Tu X 70 S ribosomal complex with dimethylsuberimidate. EF-Tu has been found to be in contact with the 50 S subunit in the region of the L7/L12 stalk and with the 30 S subunit in the upper part of its body on the side opposite the top of the ledge (the platform). The EF-Tu position on a model of the 70 S ribosome is presented.