Conformation of ribonuclease S-protein

Pretransitional structural changes in the thermal denaturation of ribonuclease S and S protein

Two mechanisms have been proposed for the thermal unfolding of ribonuclease S (RNase S). The first is a sequential partial unfolding of the S peptide/S protein complex followed by dissociation, whereas the second is a concerted denaturation/dissociation. The thermal denaturation of ribonuclease S and its fragment, the S protein, were followed with circular dichroism and infrared spectra. These spectra were analyzed by the principal component method of factor analysis. The use of multiple spectral techniques and of factor analysis monitored different aspects of the denaturation simultaneously. The unfolding pathway was compared with that of the parent enzyme ribonuclease A (RNase A), and a model was devised to assess the importance of the dissociation in the unfolding. The unfolding patterns obtained from the melting curves of each protein imply the existence of multiple intermediate states and/or processes. Our data provide evidence that the pretransition in the unfolding of ribonuclease S is due to partial unfolding of the S protein/S peptide complex and that the dissociation occurs at higher temperature. Our observations are consistent with a sequential denaturation mechanism in which at least one partial unfolding step comes before the main conformational transition, which is instead a concerted, final unfolding/dissociation step.

Conformational requirements for glycoprotein reglucosylation in the endoplasmic reticulum

Newly synthesized glycoproteins interact during folding and quality control in the ER with calnexin and calreticulin, two lectins specific for monoglucosylated oligosaccharides. Binding and release are regulated by two enzymes, glucosidase II and UDP-Glc:glycoprotein:glycosyltransferase (GT), which cyclically remove and reattach the essential glucose residues on the N-linked oligosaccharides. GT acts as a folding sensor in the cycle, selectively reglucosylating incompletely folded glycoproteins and promoting binding of its substrates to the lectins. To investigate how nonnative protein conformations are recognized and directed to this unique chaperone system, we analyzed the interaction of GT with a series of model substrates with well defined conformations derived from RNaseB. We found that conformations with slight perturbations were not reglucosylated by GT. In contrast, a partially structured nonnative form was efficiently recognized by the enzyme. When this form was converted back to a nativelike state, concomitant loss of recognition by GT occurred, reproducing the reglucosylation conditions observed in vivo with isolated components. Moreover, fully unfolded conformers were poorly recognized. The results indicated that GT is able to distinguish between different nonnative conformations with a distinct preference for partially structured conformers. The findings suggest that discrete populations of nonnative conformations are selectively reglucosylated to participate in the calnexin/calreticulin chaperone pathway.

Native-state hydrogen-exchange studies of a fragment complex can provide structural information about the isolated fragments

Ordered protein complexes are often formed from partially ordered fragments that are difficult to structurally characterize by conventional NMR and crystallographic techniques. We show that concentration-dependent hydrogen exchange studies of a fragment complex can provide structural information about the solution structures of the isolated fragments. This general methodology can be applied to any bimolecular or multimeric system. The experimental system used here consists of Ribonuclease S, a complex of two fragments of Ribonuclease A. Ribonuclease S and Ribonuclease A have identical three-dimensional structures but exhibit significant differences in their dynamics and stability. We show that the apparent large dynamic differences between Ribonuclease A and Ribonuclease S are caused by small amounts of free fragments in equilibrium with the folded complex, and that amide exchange rates in Ribonuclease S can be used to determine corresponding rates in the isolated fragments. The studies suggest that folded RNase A and the RNase S complex exhibit very similar dynamic behavior. Thus cleavage of a protein chain at a single site need not be accompanied by a large increase in flexibility of the complex relative to that of the uncleaved protein.