Final answer:
In coiled-coils, subunits are held together primarily by hydrophobic interactions, where nonpolar amino acids on one side of an alpha-helix align and intertwine with another, forming a stable structure.
Step-by-step explanation:
Coiled-coils are a structural motif in proteins where 2-7 alpha-helices wind around each other to form a supercoil. The stability of coiled-coils is predominantly due to hydrophobic interactions. These interactions occur when nonpolar amino acids in the alpha-helices are located on one side of the helix, causing them to associate tightly together and avoid water. This alignment leads to the intertwining of the helices, with their hydrophobic side chains interlocking like the teeth of a zipper. Although other types of interactions, such as ionic bonding, hydrogen bonding, and disulfide linkages also play roles in stabilizing protein tertiary structure, within the context of coiled-coils, the hydrophobic interactions are the key factor that hold the subunits together.