MISFOLDING AND DISEASES

       In 1994, Sosnick et al. demonstrated that cytochrome c is able to fold much more quickly than previously thought when specific interactions in a proposed intermediate state are removed.This opened up the possibility that intermediates in protein folding are not necessarily integral to the folding process, and may not be desirable for the optimisation of the rate of the process.Such intermediates may act as kinetic traps, and slow the rate of folding, as interactions involved in the stabilisation of the intermediate may need to be undone before the protein can undergo the remainder of the folding process. Fersht suggests that such intermediates may be an unavoidable consequence of increased size, and thus the possibility of nucleation of folding at more than one site.
       This ``new view'' of protein folding is beginning to be associated with ideas on protein misfolding and disease. It is obvious that the structure of a protein and its ability to carry out its correct function are very tightly linked such that small structural defects can lead to a number of protein folding diseases. These include genetic diseases such as cystic fibrosis and sickle cell anaemia, which are caused by single residue deletion and mutation respectively, rendering the protein incapable of its normal function.
       More recently a number of diseases have been linked to protein folding problems which lead to the build up of insoluble protein plaques in the brain or other organs. The protein plaques are found to consist of amyloid fibrils - polymerised cross- beta-sheet structures with the beta-strands arranged perpendicular to the long axis of the fibre.These diseases include prion diseases such as bovine spongiform encephalopathy (BSE) and its human equivalent Creutzfeld-Jakob disease (CJD), and also Alzheimer's disease, Parkinson's disease and type II (non-insulin dependent) diabetes. In all of the above cases the protein isolated from the plaques is found to be coded for by the host genome although the natural functional roles are often not known. In the case of the prion diseases, the protein isolated from the amyloid plaques is the PrP protein, and the structures of both mouse and hamster PrP-C (the normal cellular form) have been solved.The sequences of PrP-C and PrP-Sc (from amyloid plaques) are found to be identical and a purely structural change is thought to cause aggregation and fibrillogenesis. In 1997, Prusiner showed that the PrP-Sc form of the protein was the sole cause of infection in the prion diseases,and proposed a mechanism for infection. Addition of PrP-Sc from one source can cause disease, but the plaques contain PrP-Sc from the host, not the infectious particle. It is therefore believed that the PrP-Sc is able to catalyse the conversion from PrP-C to PrP-Sc by a templating mechanism.
      The ability of PrP to exist in two stable forms seems contrary to the long-held belief that a protein sequence codes for a single unique structure. However, recent studies have suggested that many other proteins undergo fibrillogenesis under certain conditions. Although not associated with any known disease, work in the Dobson group has shown that the SH3 domain of PI3 kinase forms amyloid-like fibrils under partially denaturing conditions.Similar phenomena have been observed for proteins as diverse as lysozyme, transthyretin,alpha-synuclein,immunoglobulins,microglobulins,and designed proteins such as the beta-sandwich family of betabellins.The observation of misfolded states for a large range of proteins has lead some to suggest that a major problem for protein folding pathways is avoiding these misfolded states. This ties in well with the folding funnel theory of protein folding with the suggestion that these misfolded states may simply arise from the natural frustration in the folding energy landscape, and so may be a far more ubiquitous phenomenon than previously thought. These processes have refocussed attention on our present lack of understanding of the protein folding problem, despite decades of study. The importance of extended beta-sheets in the aggregated protein masses causing these diseases also increases interest in the factors involved in the formation and stability of beta-sheet systems.