  We develop a "hybrid" model of the proton inelastic structure functions
applicable for a wide region of invariant masses of produced states $W$ and
invariant momentum transfer $Q$ including deep inelastic scattering (DIS),
nucleon resonance production as well as the region close to inelastic
threshold. In the DIS region, we compute the structure functions in terms of
the parton distributions together with higher-twist corrections which are
determined from a global QCD fit. The resonant part is addressed in terms of
the Breit-Wiegner contributions from five states: $\Delta(1232)$ resonance,
$N(1440)$ Roper resonance, and three effective resonances describing the second
and third resonance regions. The couplings of the nucleon resonances to photon
are described in terms of helicity amplitudes. The nonresonant background is
treated in terms of a smooth extrapolation of the DIS structure functions to
low-$W$ and low-$Q$ values with a proper inelastic threshold behavior and the
real photon limit $Q^2=0$ constrained by the photoproduction data. We
independently treat the transverse, $F_T$, and the longitudinal, $F_L$,
structure function and fix the model parameters from a global analysis of the
electron-proton differential cross section data together with data on the total
photoproduction cross section. We demonstrate a very good performance of the
model by comparing our predictions with data on cross sections and the
structure functions $F_2$ and $R=F_L/F_T$ in the resonance and the DIS region.
