Final answer:
The predominant secondary structure in intermediate filament proteins is the coiled-coil, a rope-like configuration providing structural stability, formed by intertwined alpha helices.
Step-by-step explanation:
The most prevalent secondary structure observed in the proteins of intermediate filaments is the coiled-coil. This secondary structure is characterized by a stable, twisted rope-like configuration and is particularly important in providing structural stability and functionality to intermediate filament proteins such as keratin. A coiled-coil structure is formed by two or more alpha helices wrapping around each other, which is a particular feature of fibrillar proteins, unlike the alpha helix or beta sheet that are commonly found in many other types of proteins.