We investigate the role of different mesoscopic interactions (Coulomb, charge regulation, and ion-dipole "surface patch" effects) on the binding of bovine serum albumin (BSA) and beta-lactoglobulin (BLG) to a cationic gold nanoparticle (TTMA+). The results demonstrate that the charge-regulation mechanism plays a vital role for selectivity of protein-nanoparticle complexation at low salt concentration. At slightly higher ionic strengths, charge-dipole effects are the dominating driving force. Thus, very small variations in salt concentration strongly influence the origin of complexation.