The optical conductivity of a heterostructure formed by a commensurate stacking of graphene and a topological insulator (TI) is investigated using the Kubo formalism. Both the intra- and interband AC conductivities are found to be sensitive to the graphene-TI coupling. The direct interband transition in graphene which is the origin of the universal conductance is forbidden due to the topological nature is the coupling. Furthermore, the graphene-TI coupling gives rise to additional broken symmetries, resulting in both the inter- and intraband conductivity to be reduced in the graphene-TI heterostructure. By varying the Fermi energy of the heterostructure, the band that gives the largest contribution changes, which in turn affects the overall electronic transport.