Molecular basis of calmodulin tethering and Ca2+-dependent inactivation of L-type Ca2+ channels.
Ca(2+)-dependent inactivation (CDI) of L-type Ca(2+) channels plays a critical role in controlling Ca(2+) entry and downstream signal transduction in excitable cells. Ca(2+)-insensitive forms of calmodulin (CaM) act as dominant negatives to prevent CDI, suggesting that CaM acts as a resident Ca(2+) sensor. However, it is not known how the Ca(2+) sensor is constitutively tethered. We have found that the tethering of Ca(2+)-insensitive CaM was localized to the C-terminal tail of alpha(1C), close to the CDI effector motif, and that it depended on nanomolar Ca(2+) concentrations, likely attained in quiescent cells. Two stretches of amino acids were found to support the tethering and to contain putative CaM-binding sequences close to or overlapping residues previously shown to affect CDI and Ca(2+)-independent inactivation. Synthetic peptides containing these sequences displayed differences in CaM-binding properties, both in affinity and Ca(2+) dependence, leading us to propose a novel mechanism for CDI. In contrast to a traditional disinhibitory scenario, we suggest that apoCaM is tethered at two sites and signals actively to slow inactivation. When the C-terminal lobe of CaM binds to the nearby CaM effector sequence (IQ motif), the braking effect is relieved, and CDI is accelerated.
Pitt
G S
GS
Department of Molecular and Cellular Physiology, Stanford University Medical School, Stanford, California 94305, USA. gp2004@columbia.edu
Zühlke
R D
RD
Hudmon
A
A
Schulman
H
H
Reuter
H
H
Tsien
R W
RW
D020816:Amino Acid Motifs; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology*; D020746:Calcium Channels, L-Type / Q000737:chemistry* / Q000502:physiology; D002147:Calmodulin / Q000737:chemistry*; D014981:Xenopus
D020746:Calcium Channels, L-Type; D002147:Calmodulin; D002118:Calcium