The charge of proteins is determined by the pKa of its amino acid side chains, and the terminal amino acid and carboxylic acid.  Proteins  with isoelectric point (pI) above physiological conditions have a positive charge and proteins with pI below physiological conditions have a negative charge.  The net charge of the protein, determined by the sum charge of its constituents, results in electrophoretic migration in a physiologic electric field.  These effects are short-range because of the high di-electric constant of water, however, once the protein is close to a charged surface, electrostatic coupling becomes the dominant force.