The V-type H(+)-ATPases are similar to the F-type ATP synthases in their structure and functional mechanism. They hydrolyze ATP coupled with proton translocation across a membrane, but in some archaea and eubacteria they also synthesize ATP in the reverse reaction. The C subunit is one of the components of the membrane-bound V(0) moiety of V-type ATPases. The C subunit of V-type H(+)-ATPase from Thermus thermophilus was crystallized in a monoclinic form and its crystal structure was determined at 1.85 A resolution by the MAD method using selenomethionyl protein. The structure has a cone (tapered cylinder) shape consisting of only two types of helix (long and short) as secondary-structure elements. The molecule is divided into three similar domains, each of which has essentially the same topology. On the basis of the structural features and molecular-surface charge distribution, it is suggested that the bottom side of the C subunit is a possible binding site for the V(0) proteolipid L-subunit ring and that the C subunit might function as a spacer unit between the proteolipid L-subunit ring and the rotating V(1) central shaft.