  The polarization dependence of the electromagnetic vertex for polarized
proton is discussed. Such a dependence is shown to be a result of gluon
polarization and gluon interactions with the constituents within proton. One
more form factor is introduced to give such a dependence in the electromagnetic
vertex for proton. Due the polarization dependence of the vertex, a nonzero
single-spin asymmetry for unpolarized lepton scattering off transversely
polarized proton is predicted.
