We present a 1-loop toroidal membrane winding sum reproducing the conjectured M -theory, four-graviton, eight derivative, R 4 amplitude. The U -duality and toroidal membrane world-volume modular groups appear as a Howe dual pair in a larger, exceptional, group. A detailed analysis is carried out for M -theory compactified on a 3-torus, where the target-space SL(3, Z) × SL(2, Z) U -duality and SL(3, Z) world-volume modular groups are embedded in E 6(6) (Z). Unlike previous semi-classical expansions, U -duality is built in manifestly and realized at the quantum level thanks to Fourier invariance of cubic characters. In addition to winding modes, a pair of new discrete, flux-like, quantum numbers are necessary to ensure invariance under the larger group. The action for these modes is of Born-Infeld type, interpolating between standard Polyakov and Nambu-Goto membrane actions. After integration over the membrane moduli, we recover the known R 4 amplitude, including membrane instantons. Divergences are disposed of by trading the non-compact volume integration for a compact integral over the two variables conjugate to the fluxes -a constant term computation in mathematical parlance. As byproducts, we suggest that, in line with membrane/fivebrane duality, the E 6 theta series also describes five-branes wrapped on T 6 in a manifestly U-duality invariant way. In addition we uncover a new action of E 6 on ten dimensional pure spinors, which may have implications for ten dimensional super Yang-Mills theory. An extensive review of SL(3) automorphic forms is included in an Appendix.