The structure and harmonic and anharmonic IR spectra of the protonated water dimer (PWD) were calculated in C-1,C- C-2.., and C-s, symmetry at the MP4/acc-pVTZ level of theory. We found that structure and IR spectra are practically identical in C-2 and C-1 symmetry, demonstrating that an equilibrium C-1 configuration of the PWD is not realized. Anharmonic coupling of the shared proton stretching-vibration with all other modes in the PWD in C-2 and C-s symmetry was the focus of this investigation. For this purpose, 28 two-dimensional potential energy surfaces (2D PES) were built at the MP4/acc-pVTZ level of theory and the corresponding vibrational Schrodinger equations were solved using the DVR method. Differences in the coupling of the investigated mode with other modes in the C-2 and C-s configurations, along with some factors that determine the red-or blue-shift of the stretching vibration frequency, were analyzed. We obtained a rather reasonable value of the stretching frequency of the bridging proton (1058.4 cm(-1)) unperturbed by Fermi resonance. The Fermi resonance between the fundamental vibration v7 and the combined vibration v(2) + v(6) of the same symmetry was analyzed through anharmonic second order perturbation theory calculations, as well as by 3D PES constructed using Q(2), Q(6), and Q(7) as normal coordinates. A significant (up to 50%) transfer of intensity from the fundamental vibration to the combined one was found. We have estimated the frequency of the bridging proton stretching vibration in the C-s configuration of the PWD based on calculations of the intrinsic anharmonicity and anharmonic double modes interactions at the MP4/acc-pVTZ level of theory (1261 cm(-1)).