Kinetic stabilisation of a modular protein by domain interactions

Protein S, a two-domain spore coat protein from Myxococcus xanthus, is structurally related to eye-lens Pr crystallins. No natural monomeric one-domain member of this protein superfamily is known. To determine the stability of the single domains and to explain the ubiquitous domain duplication, the isolated domains of protein S were constructed. The N-domain is thermodynamically more stable than the C-domain. In intact protein S, domain interactions lead to an apparent decrease in stability of the N-terminal domain, whereas the C-terminal domain is stabilised. In contrast, unfolding kinetics of both domains are decreased 100-fold due to interactions in the complete molecule.