Manipulation of unfolding-induced protein aggregation by peptides selected for aggregate-binding ability through phage display library screening

Evidence of native-like substructure(s) in polypeptide chains of carbonic anhydrase deposited into insoluble aggregates during thermal unfolding

A non-specific protease, subtilisin, was used to probe the existence of folded structure in thermal aggregates of bovine carbonic anhydrase-II (BCA). BCA aggregates and native BCA were subjected to proteolysis and electrophoretic analyses which revealed the accumulation of polypeptide fragments of similar size, indicating survival of similar sections of folded structure burying peptide bonds away from scission in the two samples. N-terminal sequencing revealed that the termini of size-matched fragments from the two samples were either identical, or located very close to each other, and predominantly on the surface of the 3-dimensional structure of native BCA. The susceptibility to proteolysis of very nearly the same sites in the two samples suggests that native-like elements of structure survive within BCA aggregates. The finding that thermal aggregation can involve interactions among molecules retaining elements of native-like structure, suggests that complete chain unfolding may not be a necessary prerequisite for all aggregation.

Peptide scanning-based identification of regions of gamma-II crystallin involved in thermal aggregation: evidence of the involvement of structurally analogous, helix-containing loops from the two double Greek key domains of the molecule

Gamma crystallin is one of three structural proteins present in great abundance in the fiber cells of the vertebrate eye lens. The protein displays a tendency to aggregate readily in the course of heating, cooling, being exposed to ultraviolet radiation, or rapid refolding. To investigate the molecular mechanisms underlying such aggregation, we have employed a peptide-scanning approach aimed at identifying regions of bovine gamma-II crystallin that may be involved in intermolecular interactions leading to aggregation, using assays that measure the competitive inhibition of such aggregation by reagents drawn from a group of contiguous (overlapping) peptides derived from the sequence of the protein itself. Our results suggest that two regions, comprising residues 61-74, and 145-159, play key roles in aggregative interactions. Intriguingly, the two regions (each containing a solvent-exposed, single-turn helix in the native structure) are located in structurally analogous positions in the two homologous double Greek key (beta sheet) domains of the protein, suggesting that helix-strand conversions may operate to facilitate intermolecular beta sheet interactions during aggregation.