  We analytically show that a gap is induced around the Dirac point in the
electronic spectrum of a previously metallic nanotube, in the presence of
electric and magnetic fields perpendicular to the tube axis. For realistic
values of the fields, a gap of at least a few meV can appear. Despite the
quasi-one dimensional nature of the system, the gapped state is associated with
a non-zero topological invariant and supports a Hall effect. This is revealed
when the flux through the tube is varied by one flux quantum, which leads to
exactly one electron per spin being transported between the ends of the tube.
