The electrical properties of single-wall C, BN, and BC3 nanotubes in ideally rolled-up forms show a wide spectrum from truly metals to large band gap semiconductors. In the presence of radial deformations that collapse tubes, the electrical properties are severely modified such that metals turn into semiconductors and vice versa. Based on first-principles pseudopotential calculations, we find that metallic C nanotubes have a finite band gap if radial deformations break all mirror symmetries of the tubes, and that original finite gaps (similar to 0.5 eV) of semiconducting C and BC3 tubes are closed by collapsing deformations. In BN tubes, band gaps can be tuned in the range 2-5 eV. On the other hand, the band gaps of armchair BN and zigzag BC3 nanotubes are found to be insensitive to radial deformations. These new findings can be applied to design new types of nanotube-based functional devices using radial deformations. (c) 2001 Elsevier Science B.V. All rights reserved.