  We derive diffusion equations, which describe spin-charge coupled transport
on the helical metal surface of a three-dimensional topological insulator. The
main feature of these equations is a large magnitude of the spin-charge
coupling, which leads to interesting and observable effects. In particular, we
predict a new magnetoresistance effect, which manifests in a nonohmic
correction to a voltage drop between a ferromagnetic spin-polarized electrode
and a nonmagnetic electrode, placed on top of the helical metal. This
correction is proportional to the cross-product of the spin polarization of the
ferromagnetic electrode and the charge current between the two electrodes. We
also demonstrate tunability of this effect by applying a gate voltage, which
makes it possible to operate the proposed device as a transistor.
