Protein 3 may attach the radial spoke to the outer doublet microtubule or is required to form a stable spoke structure. Flagellar radial spokes contribute to the regulation of dynein arm activity and thus the pattern of flagellar bending. They consist of a thin stalk, which is attached to the a subfiber of the outer doublet microtubule, and a bulbous head, which is attached to the stalk and appears to interact with the projections from the central pair of microtubules. Interacts with FAP91. Radial spoke. Protein 3 is one of the 5 radial spoke proteins that are phosphorylated. Protein 3a might only differ from protein 3 in being unphosphorylated. Belongs to the flagellar radial spoke RSP3 family.