Responsive interfaces are often realized by polymer films that change their structure and properties upon changing the pH-value， ionic strength or temperature. Here， we present a bioresponsive interfacial structure that is based on a protein， calmodulin (CaM)， which undergoes a huge conformational change upon ligand binding. At first， we characterize the conformational functionality of a double Cys mutant of CaM by small-angle X-ray scattering (SAXS) and Fourier transform infrared (FTIR) spectroscopy. The CaM mutant is then used to cross-link poly(ethylene glycol) (PEG) chains， which are bound covalently to a supporting planar Si surface. These films are characterized by X-ray reflectometry (XR) in a humidity chamber providing full hydration. It is well known that Ca-saturated holo-CaM binds trifluoperazine (TFP) and changes its conformation from an open， dumbbell-shaped to a closed， globular one in solution. At the interface， we observe an increase of the PEG-CaM film thickness， when TFP is binding and inducing the closed conformation， whereas the removal of Ca-ions and a concomitant release of TFP is associated with a decrease of the film thickness. This toggling of the film thickness is largely reversible. In this way， a structural change of the interface is achieved via protein functionality which has the advantage of being selective for ligand molecules without changing the environmental conditions in a harsh way via physico-chemical parameters.