The M4 segment in heteromeric AMPAR assembly. A, Whole-cell currents at −70 mV in HEK 293 cells transfected with either GluA1/GluA2(R) or GluA1/GluA2(R)-ΔM4 (1:1 ratios). Solid bar indicates glutamate (3 mm) application. B, Mean current amplitudes (±SEM; n > 5). *p < 0.05 [values significantly different from GluA1/GluA2(R); Student's t test]. C–E, Current–voltage relationships (−100 to +100 mV) for HEK 293 cells transfected with indicted cDNA. F, BN-PAGE of transfected HEK 293 cells probed with either anti-A1 (left) or anti-A2 (right).