Voltage-gated potassium (Kv) channels containing α-subunits of the Kv2 subfamily mediate delayed rectifier currents in excitable cells. Channels formed by Kv2.1 α-subunits inactivate from open- and closed states with both forms of inactivation serving different physiological functions. Here we show that open- and closed-state inactivation of Kv2.1 can be distinguished by the sensitivity to intracellular tetraethylammonium and extracellular potassium and lead to the same inactivated conformation. The functional properties of Kv2.1 are regulated by its association with modulatory α-subunits (Kv5, Kv6, Kv8, and Kv9). For instance, Kv9.3 changes the state preference of Kv2.1 inactivation by accelerating closed-state inactivation and inhibiting open-state inactivation. An N-terminal regulatory domain (NRD) has been suggested to determine the function of the modulatory α-subunit Kv8.1. However, when we tested the NRD of Kv9.3, we found that the functional properties of chimeric Kv2.1 channels containing the NRD of Kv9.3 (Kv2.1NRD) did not resemble those of Kv2.1/Kv9.3 heteromers, thus questioning the role of the NRD in Kv9 subunits. A further region of interest is a PXP motif in the sixth transmembrane segment. This motif is conserved among all α-subunits of the Kv1, Kv2, Kv3, and Kv4 subfamilies, whereas the second proline is not conserved in any modulatory α-subunit. Exchanging this proline in Kv2.1 for the corresponding residue of Kv9.3 resulted in channels (Kv2.1-P410T) that show all hallmarks of the regulation of Kv2.1 by Kv9.3. The effect prevailed in heteromeric channels following co-expression of Kv2.1-P410T with Kv2.1. These data suggest that the alteration of the PXP motif is an important determinant of the regulatory function of modulatory α-subunits.