Using ab initio total energy calculations we design a very thin semiconducting nanotube of germanium with a direct band gap by encapsulation of Mo or W. This finding is an outcome of studies of assemblies of Ge 18Nb 2 clusters into nanotubes. The infinite Nb-doped nanotube is metallic. However, the electronic structure has a significant gap above the Fermi level. When Nb is replaced by a Z+1 element such as Mo or W, it leads to the formation of a semiconducting nanotube. The atomic structure of these nanotubes is based on a novel alternate prism and antiprism stacking of hexagonal rings of germanium. Such an arrangement is optimal for Ge 18M 2 (M=Nb, Mo and W) clusters that serve as the building blocks of nanotubes. These results demonstrate that by just changing the M atom in the growth process, we can form metallic, semiconducting and n or p types of nanotubes, opening new possibilities for nanoscale devices. {\textcopyright}2005 The American Physical Society.