Polymerization mechanism of polypeptide chain aggregation.

The misfolding of polypeptide chains and aggregation into the insoluble inclusion body state is a serious problem for biotechnology and biomedical research. Developing a rational strategy to control aggregation requires understanding the mechanism of polymerization. We investigated the in vitro aggregation of P22 tailspike polypeptide chains by classical light scattering, nondenaturing gel electrophoresis, two-dimensional polyacrylamide gel electrophoresis (PAGE), and computer simulations. The aggregation of polypeptide chains during refolding occurred by multimeric polymerization, in which two multimers of any size could associate to form a larger aggregate and did not require a sequential addition of monomeric subunits. The cluster-cluster polymerization mechanism of aggregation is an important determinant in the kinetic competition between productive folding and inclusion body formation. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 333-343, 1997.

Conformation of P22 tailspike folding and aggregation intermediates probed by monoclonal antibodies.

The partitioning of partially folded polypeptide chains between correctly folded native states and off-pathway inclusion bodies is a critical reaction in biotechnology. Multimeric partially folded intermediates, representing early stages of the aggregation pathway for the P22 tailspike protein, have been trapped in the cold and isolated by nondenaturing polyacrylamide gel electrophoresis (PAGE) (speed MA, Wang DIC, King J. 1995. Protein Sci 4:900-908). Monoclonal antibodies against tailspike chains discriminate between folding intermediates and native states (Friguet B, Djavadi-Ohaniance L, King J, Goldberg ME. 1994. J Biol Chem 269:15945-15949). Here we describe a nondenaturing Western blot procedure to probe the conformation of productive folding intermediates and off-pathway aggregation intermediates. The aggregation intermediates displayed epitopes in common with productive folding intermediates but were not recognized by antibodies against native epitopes. The nonnative epitope on the folding and aggregation intermediates was located on the partially folded N-terminus, indicating that the N-terminus remained accessible and nonnative in the aggregated state. Antibodies against native epitopes blocked folding, but the monoclonal directed against the N-terminal epitope did not, indicating that the conformation of the N-terminus is not a key determinant of the productive folding and chain association pathway.

Specific aggregation of partially folded polypeptide chains: the molecular basis of inclusion body composition.

During expression of many recombinant proteins, off-pathway association of partially folded intermediates into inclusion bodies competes with productive folding. A common assumption is that such aggregation reactions are nonspecific processes. The multimeric intermediates along the aggregation pathway have been identified for both the P22 tailspike and P22 coat protein. We show that for a mixture of proteins refolding in vitro, folding intermediates do not coaggregate with each other but only with themselves. This indicates that aggregation occurs by specific interaction of certain conformations of folding intermediates rather than by nonspecific coaggregation, providing a rationale for recovering relatively pure protein from the inclusion body state.

Multimeric intermediates in the pathway to the aggregated inclusion body state for P22 tailspike polypeptide chains.

The failure of newly synthesized polypeptide chains to reach the native conformation due to their accumulation as inclusion bodies is a serious problem in biotechnology. The critical intermediate at the junction between the productive folding and the inclusion body pathway has been previously identified for the P22 tailspike endorhamnosidase. We have been able to trap subsequent intermediates in the in vitro pathway to the aggregated inclusion body state. Nondenaturing gel electrophoresis identified a sequential series of multimeric intermediates in the aggregation pathway. These represent discrete species formed from noncovalent association of partially folded intermediates rather than aggregation of native-like trimeric species. Monomer, dimer, trimer, tetramer, pentamer, and hexamer states of the partially folded species were populated in the initial stages of the aggregation reaction. This methodology of isolating early multimers along the aggregation pathway was applicable to other proteins, such as the P22 coat protein and carbonic anhydrase II.