Mutational Analysis of the Downhill Folding Protein gpW: Towards Tuning Stability of a Molecular Rheostat Candidate

A popular convention derived from early experimental evidence of single-domain proteins pointed towards a common mechanism of achieving their native three-dimensional structure. Concomitantly, protein function has been linked to the simple two-state approach wherein the folded state is associated with its biologically active conformation. The more recent discovery of downhill protein folding has provided a sharp contrast to this simple treatment. A catalog of qualitative signatures has been developed to distinguish between these different folding mechanisms at the two extremes (downhill and two-state). Additionally, the introduction of physical models to measure the protein folding ensemble allows the direct measurement of both the thermodynamic and kinetic barrier heights. The end result of such a quantitative approach is a more distinct partition separating the two scenarios. This methodology has been applied to the bacteriophage lambda protein gpW (gene product W) where a clear assessment of its folding behavior has been obtained. GpW is involved at the connector region at which assembly of bacteriophage heads and tails occurs. Without gpW, infectious virions are incapable of forming. This protein is also suspected of having a role in DNA binding and packaging. Contrary to its expected two-state behavior, gpW folds with a marginal barrier (< 3 RT) as determined through an examination of the thermodynamic and kinetic behaviors. Possessing a novel fold and being an independently folding protein, gpW represents the first experimentally characterized downhill folder that is not a domain of a larger complex and is known to perform a specific function. This is a clear diversion from the standpoint of a single macrostate being responsible for function and places a significant emphasis on investigating the functional role of downhill folding. With the function of performing multiple duties, gpW is an excellent candidate for functional studies of downhill folding