  We present a systematic study of the proton spin structure in terms of
measurable parton distributions. For a transversely-polarizedproton, we derive
a polarization sum rule from the leading generalized parton distributions
appearing in hard exclusive processes. For a longitudinally-polarized proton,
we obtain a helicity decomposition from well-known quark and gluon helicity
distributions and orbital angular-momentum contributions. The latter is shown
to be related to measurable subleading generalized parton distributions and
quantum-phase space Wigner distributions.
