Both N- and C-lobes of calmodulin are required for Ca2+-dependent regulations of CaV1.2 Ca2+ channels

We investigated the concentration- and Ca2+-dependent effects of CaM mutants, CaM12 and CaM34, in which Ca2+-binding to its N- and C-lobes was eliminated, respectively, on the CaV1.2 Ca2+ channel by inside-out patch clamp in guinea-pig cardiomyocytes. Both CaM12 and CaM34 (0.7–10 μM) applied with 3 mM ATP produced channel activity after “rundown”. Concentration–response curves were bell-shaped, similar to that for wild-type CaM. However, there was no obvious leftward shift of the curves by increasing [Ca2+], suggesting that both functional lobes of CaM were necessary for the Ca2+-dependent shift. However, channel activity induced by the CaM mutants showed Ca2+-dependent decrease, implying a Ca2+ sensor existing besides CaM. These results suggest that both N- and C-lobes of CaM are required for the Ca2+-dependent regulations of CaV1.2 Ca2+ channels.