Stimuli-sensitive polymer hydrogels, which swell or shrink in response to changes in the environmental conditions, have been extensively investigated and used as ＇smart＇ biomaterials and drug-delivery systems(1,2). Most of these responsive hydrogels are prepared from a limited number of synthetic polymers and their derivatives, such as copolymers of (meth)acrylic acid, acrylamide and N-isopropyl acrylamide(3-12). Water-soluble synthetic polymers have also been crosslinked with molecules of biological origin, such as oligopeptides(13) and oligodeoxyribonudeotides(14), or with intact native proteins(15). Very often there are several factors influencing the relationship between structure and properties in these systems, making it difficult to engineer hydrogels with specified responses to particular stimuli. Here we report a hybrid hydrogel system assembled from water-soluble synthetic polymers and a well-defined protein-folding motif, the coiled coil. These hydrogels undergo temperature-induced collapse owing to the cooperative conformational transition of the coiled-coil protein domain. This system shows that well-characterized water-soluble synthetic polymers can be combined with well-defined folding motifs of proteins in hydrogels with engineered volume-change properties(16,17).