Molecular chaperones in protein folding and translocation

Chaperonin cpn60 and heat shock protein hsp70 couple their ATPase cycles to the binding and dissociation of non-native proteins. cpn60 is a cylindrical tetradecamer that uses a co-protein (cpn10) and both positive and negative cooperativity to alter the properties of its two voluminous protein-binding chambers in an alternating, asymmetric cycle. In the hsp70 reaction cycle, short segments of polypeptide bind rapidly and weakly to the ATP state, so triggering hydrolysis and consequent stabilization of the complex. Co-proteins of the hsp40 family enhance this partial reaction, whereas nucleotide exchange factors destabilize the product. The individual steps in the two energy transducing mechanisms have only recently been elucidated and provide us with a more detailed picture of the way in which these chaperones can influence the folding, assembly and translocation of protein structures in the cell.

Ligand-induced conformational changes in the apical domain of the chaperonin GroEL

Although the role of nucleotides in the catalytic cycle of the GroESL chaperonin system has been extensively studied, the molecular effects of nucleotides in modulating exposure of sites on GroEL has not been thoroughly investigated. We report here that nucleotides (ATP, ADP, or adenosine 5'-(beta, gamma-imino)triphosphate) in the presence of Mg2+ make the oligomer selectively sensitive to trypsin proteolysis in a fashion suggesting conformational changes in the monomers of one heptameric ring. The site of proteolysis in the monomer that is exposed upon nucleotide binding by the oligomer is in the apical domain (Arg-268). Further, complexes of GroEL with GroES or rhodanese display the same sensitivity to proteolysis, unlike the GroEL-GroES-rhodanese complex, which is protected from proteolysis. The influence of various cations on trypsin proteolysis is investigated to elucidate the differential effects that monovalent and divalent cations have on the oligomeric structure of the chaperonin. These results are discussed in relation to the molecular basis for the chaperonin activity.

The fates of proteins in cells

Nascent polypeptide chains are in a dangerous situation as soon as they leave their place of birth, the channel of the large ribosomal subunit: more than 20 different pathways for the degradation of proteins exist in cells. Chaperones protect and guide the young protein molecules and support their correct foldings. Targeting signals direct the proteins to the organelles of their destination. The lysosome is the site of random degradation, while the proteasome is highly selective. Although these two organelles provide the most important pathways for the degradation of long- and short-lived proteins, other pathways with roles in deciding the fate of cellular proteins must also be considered.

The chaperonin GroEL is destabilized by binding of ADP

The urea-induced dissociation and subsequent conformational transitions of the nucleotide-bound form of GroEL were studied by light scattering, 4,4'-bis(1-anilino-8- naphthalenesulfonic acid) binding, and intrinsic tyrosine fluorescence. Magnesium ion alone (10 mM) stabilizes GroEL and leads to coordination of the structural transitions monitored by the different parameters. The midpoint of the light-scattering transition that monitored dissociation of the 14-mer with bound magnesium was raised to approximately 3 M, which is considerably higher than the ligand-free form of the protein, which exhibits a transition with a midpoint at approximately 2 M urea. Binding of ADP results in destabilization of the GroEL oligomeric structure, and complete dissociation of the 14-mer in the presence of 5 mM ADP occurs at about 2 M urea with the midpoint of the transition at approximately 1 M urea. The same destabilization by ADP and stabilization by Mg2+ were seen when the conformation was followed by the intrinsic fluorescence. Complexation with the nonhydrolyzable ATP analog, 5'-adenylimidodiphosphate gave an apparent stability of the quaternary structure that was between that observed with Mg2+ and that with ADP. The ADP-bound form of the protein demonstrated increased hydrophobic exposure at lower urea concentrations than the uncomplexed GroEL. In addition, the GroEL-ADP complex is more accessible for proteolytic digestion by chymotrypsin than the uncomplexed protein, consistent with a more open, flexible form of the protein. The implication of the conformational changes to the mechanism of the GroEL function is discussed.

Inactive GroEL monomers can be isolated and reassembled to functional tetradecamers that contain few bound peptides

For the first time, it has been shown that GroEL can be converted from tetradecamers (14-mers) to monomers under conditions commonly used for the preparation of this chaperonin. The essential requirements are the simultaneous presence of nucleotides such as MgATP or MgADP and a solid-phase anion-exchange medium. The monomers that are formed are metastable in that they only reassemble to a small degree in the absence of additives. These results are in keeping with previous studies on high pressure dissociation that showed the separated monomers display conformational plasticity and can undergo conformational relaxation when relieved of the constraints of the quaternary structure in the oligomer (Gorovits, B., Raman, C. S., and Horowitz, P. M. (1995) J. Biol. Chem. 270, 2061-2066). The monomers display greatly enhanced hydrophobic exposure to the probe 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid, although they are not active in folding functions, and they are unable to form complexes with partially folded rhodanese. The monomers can be completely reassembled to 14-mers by incubation in 1 M ammonium sulfate. There is no evidence of intermediates in the reassembly process. Compared with the original oligomers, the reassembled 14-mers have (a) very low levels of polypeptide contaminants and tryptophan-like fluorescence, two problems that previously hampered spectroscopic studies of GroEL structure and function; (b) functional properties that are very similar to the original material; (c) considerably decreased hydrophobic exposure in the native state; and (d) a similar triggered exposure of hydrophobic surfaces after treatment with urea or spermidine. This study demonstrates that the quaternary structure of GroEL is more labile than previously thought. These results are consistent with suggestions that nucleotides can loosen subunit interactions and show that changes in quaternary structure can operate under conditions where GroEL function has been demonstrated.

A monomeric variant of GroEL binds nucleotides but is inactive as a molecular chaperone

The heat shock protein GroEL from Escherichia coli is a tetradecameric oligomer that facilitates the refolding of nonnative polypeptides in an ATP-hydrolysis dependent reaction. A mutant in GroEL was prepared in which lysine 3 was substituted with glutamate, which destabilizes the oligomeric structure of GroEL (Horovitz, A., Bochkareva, E.S., and Girshovich, A.S. (1993) J. Biol. Chem. 268, 9957-9959). The highly expressed and purified GroELK3E was judged to be monomeric by sedimentation equilibrium, yielding a molecular weight of 54,500, despite a weak tendency of the mutant to reversibly form higher order aggregates above 4 mg ml-1. The monomeric variant appears to be folded based on the far UV circular dichroism spectrum, which shows significant alpha-helical content, but with slight differences in conformation relative to wild-type GroEL. The increase in exposed hydrophobic surface of the monomer was probed with the dye 4,4'-bis-1-anilino-3-naphthalenesulfonate (bis-ANS). The fluorescence of bis-ANS increases approximately 150-fold in the presence of the mutant, and about 4 mol of bis-ANS bind per mol of monomer, with a binding constant of 1.6 microM. Adenosine nucleotide binding to monomeric GroELK3E resulted in considerable quenching of bis-ANS fluorescence, correlating with significant structural changes as seen in the far UV circular dichroism, and permitted the measurement of binding isotherms for ATP and ADP. Hyperbolic ATP binding isotherms yield a dissociation constant of 82 microM, about 4-fold weaker than the K0.5 for ATP seen in steady-state kinetics assays of the wild-type GroEL ATPase.A similar difference was seen for ADP binding. These results suggest that the mutation disrupts the native tetradecameric quaternary structure through conformational changes that may also weaken nucleotide binding. The monomeric mutant exhibited no chaperone activity as evidenced by a filure to inhibit or facilitate the refolding of chemically denatured enolase, an inability to refold denatured rhodanese above spontaneous levels, and a lack of binding to alpha-casein, a competitor in many chaperonin-promoted refolding reactions. Thus, the formation of assembly incompetent monomers by the lysine 3 to glutamate mutation results in a dramatic decrease in the affinity for nonnative polypeptide chains and suggests that the oligomeric nature of GroEL is crucial for its molecular chaperone function.

Temperature-induced exposure of hydrophobic surfaces and its effect on the chaperone activity of alpha-crystallin

alpha-Crystallin, the major protein of the ocular lens, is known to have extensive similarities to small heat shock proteins and to act as a molecular chaperone. The exposure of hydrophobic surfaces on alpha-crystallin was studied by fluorescence spectroscopy using the hydrophobic probe bis-ANS. Upon heating the protein undergoes a conformational transition which is associated with a marked increase in surface hydrophobicity. This transition, which occurs between approximately 38 and 50 degrees C, lacks reversibility. The increase in surface hydrophobicity correlates with the increased chaperone activity of the protein. These results indicate that hydrophobic interactions play a major role in the chaperone action of alpha-crystallin.

Exposure of hydrophobic surfaces on the chaperonin GroEL oligomer by protonation or modification of His-401

Hydrophobic exposure on the chaperonin GroEL is increased 6-10-fold after the protein is treated with the His-reactive reagent diethyl pyrocarbonate (DEP), or the solution pH is lowered to 5.5. The induced hydrophobic surfaces have the same 1,1'-bis(4-anilino)naphthalene-5,5'-disulfonic acid (bis-ANS) binding characteristics as unperturbed GroEL: a Kd approximately equal to 3.5 microM, a maximum intensity at approximately 500 nm, and an average fluorescence lifetime of approximately 8.0 ns. The pKa for the pH-induced transition is 6.6, most likely attributable to the only histidine in GroEL, His-401, located in the intermediate domain. The modification of one histidine residue per monomer upon DEP treatment is supported by the correlation between the change in the absorbance at 242 nm for the N-carbethoxyhistidyl derivative and the increase in bis-ANS fluorescence. GroEL at pH 5.5 is tetradecameric and can capture urea-denatured rhodanese and release it as active enzyme. The GroEL-rhodanese and release it as active enzyme. The GroEL-rhodanese complex is more stable to dissociation by 2.25 M urea than the complex formed at pH 7.8. We propose that His-401 is in a conformationally sensitive region such that protonation or modification can lead to increased exposure of hydrophobic surfaces capable of binding folding intermediates.