Binding and release by MBP of partially folded passenger proteins eventually results in their spontaneous folding while avoiding the JI 101 site kinetically competing self-aggregation pathway. The hydrophobic ligand-binding pocket in MBP, which is not present in other highly soluble proteins that do not function as solubility enhancers (e.g., GST), was proposed to be the locus of polypeptide binding. The phenotypes of some mutations in MBP were observed to be consistent with this model [25]. However, one might then expect that the occupation of this pocket by maltose, which results in the transition from an “open” to a “closed” complex [39], would impede solubility enhancement by MBP. Yet, at odds with this prediction, we found that the inclusion of as much as 30 mM maltose in refolding experiments did not appreciably reduce the recovery of soluble MBP fusion proteins (MBP has a KD of 1200 nM for maltose [40]). This does not necessarily rule out the intramolecular chaperone model, however, because the proposed interaction site may lie elsewhere on the surface of MBP [8].Two Pathways for the Folding of Passenger ProteinsWe have shown that there are at least two pathways to the native state for passenger proteins that have been rendered soluble by fusing them to MBP. Some proteins such as TEV protease andGFP can fold spontaneously if their propensity to form insoluble aggregates is blocked by fusing them to MBP. Other passenger proteins, exemplified by G3PDH and DHFR, depend on endogenous GroES/L to fold correctly after being solubilized by MBP. In both cases, MBP serves as a kind of “holdase” to maintain the passenger 1676428 in the absence of chaperones, the yield of active enzyme on a mole-per-mole basis is far greater as an MBP fusion protein than as a His6-GST or His6-tagged protein (Figure 2B). This contrasts with GFP and TEV protease, which exhibit similar mole-per-mole refolding yields with the various tags and therefore appear to undergo spontaneous rather than MBPassisted folding. The unusual behavior of DUSP14 suggests the existence of yet another possible pathway for passenger protein folding that is more directly dependent on MBP. Co-expression experiments conducted with the MBP-GFP and NusA-GFP fusion proteins in the presence of the GroE3? variant unequivocally demonstrate that proteins larger than the theoretical volume of the cavity formed by a GroEL heptamer can engage in productive folding interactions with the chaperonin. Moreover, a cell-wide survey of GroEL/S clients identified several proteins larger than 60 kDa [41,42]. It is now generally accepted that these large substrates/clients utilize a so-called “trans” mechanism in which they occupy one of the two cavities in the back-to-back dimer of GroEL heptamers while the other empty cavity binds the co-chaperonin GroES and ATP, enabling conformational changes to be propagated from one cavity to the other [43,44]. One needs to bear in mind that even though we have emphasized the interaction of passenger proteins with GroEL/S, it is also possible that the chaperonin interacts with MBP as well [45]. We have found GroEL co-purifying with MBP on.Binding and release by MBP of partially folded passenger proteins eventually results in their spontaneous folding while avoiding the kinetically competing self-aggregation pathway. The hydrophobic ligand-binding pocket in MBP, which is not present in other highly soluble proteins that do not function as solubility enhancers (e.g., GST), was proposed to be the locus of polypeptide binding. The phenotypes of some mutations in MBP were observed to be consistent with this model [25]. However, one might then expect that the occupation of this pocket by maltose, which results in the transition from an “open” to a “closed” complex [39], would impede solubility enhancement by MBP. Yet, at odds with this prediction, we found that the inclusion of as much as 30 mM maltose in refolding experiments did not appreciably reduce the recovery of soluble MBP fusion proteins (MBP has a KD of 1200 nM for maltose [40]). This does not necessarily rule out the intramolecular chaperone model, however, because the proposed interaction site may lie elsewhere on the surface of MBP [8].Two Pathways for the Folding of Passenger ProteinsWe have shown that there are at least two pathways to the native state for passenger proteins that have been rendered soluble by fusing them to MBP. Some proteins such as TEV protease andGFP can fold spontaneously if their propensity to form insoluble aggregates is blocked by fusing them to MBP. Other passenger proteins, exemplified by G3PDH and DHFR, depend on endogenous GroES/L to fold correctly after being solubilized by MBP. In both cases, MBP serves as a kind of “holdase” to maintain the passenger 1480666 proteins in an aggregation-resistant form that either permits spontaneous folding to occur or affords access to molecular chaperones. Among the passenger proteins examined in the present study, DUSP14 represents a unique case because its folding pathway differs in at least one respect from those described above. Although DUSP14 folds in vitro 1676428 in the absence of chaperones, the yield of active enzyme on a mole-per-mole basis is far greater as an MBP fusion protein than as a His6-GST or His6-tagged protein (Figure 2B). This contrasts with GFP and TEV protease, which exhibit similar mole-per-mole refolding yields with the various tags and therefore appear to undergo spontaneous rather than MBPassisted folding. The unusual behavior of DUSP14 suggests the existence of yet another possible pathway for passenger protein folding that is more directly dependent on MBP. Co-expression experiments conducted with the MBP-GFP and NusA-GFP fusion proteins in the presence of the GroE3? variant unequivocally demonstrate that proteins larger than the theoretical volume of the cavity formed by a GroEL heptamer can engage in productive folding interactions with the chaperonin. Moreover, a cell-wide survey of GroEL/S clients identified several proteins larger than 60 kDa [41,42]. It is now generally accepted that these large substrates/clients utilize a so-called “trans” mechanism in which they occupy one of the two cavities in the back-to-back dimer of GroEL heptamers while the other empty cavity binds the co-chaperonin GroES and ATP, enabling conformational changes to be propagated from one cavity to the other [43,44]. One needs to bear in mind that even though we have emphasized the interaction of passenger proteins with GroEL/S, it is also possible that the chaperonin interacts with MBP as well [45]. We have found GroEL co-purifying with MBP on.
