Proteins are used for many structural functions such as in the actin and myosin in muscle or as a part of the cytoskeleton scaffolding that maintains cell shape. what other main function do proteins serve? a. as enzymes to control the rate of reactions and regulate cellular activity b. as waterproof membranes to regulate the transport of molecules in and out of the cell c. as coding for genotypic expression and phenotypic traits d. as cellulose to form the major structural component of plant cell walls