Surface representation of the phosphotyrosine binding Src Homology 2 (SH2) domain of the nonreceptor tyrosine kinase v-SRC, with bound phospotyrosine-containing peptide fragment [PDB: 1SHB]. Electrostatic surface potential (distribution of charge along surface) calculated with APBS & PDB2PQR software (www.poissonboltzmann.org) was depicted as colored surface in VMD (Visual Molecular Dynamics software). The color scale indicates regions of positive charge values as blue, negative charge values as red and neutral charges as white. Note the strongly positively charged surface cavity comprising the binding site for the negatively charged phosphotyrosine in the modified peptide.