The low-resolution structure and overall dimensions of the A3B3CDF complex of the A1 ATPase from Methanosarcina mazei Gö1 in solution is analyzed by synchrotron X-ray small-angle scattering. The radius of gyration and the maximum size of the complex are 5.03 ± 0.1 and 18.0 ± 0.1 nm, respectively. The low-resolution shape of the protein determined by two independent ab initio approaches has a knob-and-stalk-like feature. Its headpiece is approximately 9.4 nm long and 9.2 nm wide. The stalk, which is known to connect the headpiece to its membrane-bound Ao part, is approximately 8.4 nm long. Limited tryptic digestion of the A3B3CDF complex was used to probe the topology of the smaller subunits (C-F). Trypsin was found to cleave subunit C most rapidly at three sites, Lys20, Lys21, and Arg209, followed by subunit F. In the A3B3CDF complex, subunit D remained protected from proteolysis.