Structural basis for [Ca.sup.2+] selectivity of a voltage-gated calcium channel

• Published in: Nature (London) Vol. 505; no. 7481; pp. 56 - 62
• Main Authors: Tang, Lin, El-Din, Tamer M. Gamal, Payandeh, Jian, Martinez, Gilbert Q, Heard, Teresa M, Scheuer, Todd, Zheng, Ning, Catterall, William A
• Format: Journal Article
• Language: English
• Published: Nature Publishing Group 02.01.2014
• Subjects: Binding sites (Biochemistry); Calcium channels; Molecular structure; Properties
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12775

Voltage-gated calcium ([Ca.sub.V]) channels catalyse rapid, highly selective influx of [Ca.sup.2+] into cells despite a 70-fold higher extracellular concentration of [Na.sup.+] .How [Ca.sub.V] channels solve this fundamental biophysical problem remains unclear. Here we report physiological and crystallographic analyses of a calcium selectivity filter constructed in the homotetrameric bacterial [Na.sub.V] channel [Na.sub.V]Ab. Our results reveal interactions of hydrated [Ca.sup.2+] with two high-affinity [Ca.sup.2+] -binding sites followed by a third lower-affinity site that would coordinate [Ca.sup.2+] as it moves inward. At the selectivity filter entry, Site 1 is formed by four carboxyl side chains, which have a critical role in determining [Ca.sup.2+] selectivity. Four carboxyls plus four backbone carbonyls form Site 2, which is targeted by the blocking cations [Cd.sup.2+] and [Mn.sup.2+], with single occupancy. The lower-affinity Site 3 is formed by four backbone carbonyls alone, which mediate exit into the central cavity. This pore architecture suggests a conduction pathway involving transitions between two main states with one or two hydrated [Ca.sup.2+] ions bound in the selectivity filter and supports a 'knock-off' mechanism of ion permeation through a stepwisebinding process. The multi-ion selectivity filter of our [Ca.sub.V]Ab model establishes a structural framework for understanding the mechanisms of ion selectivity and conductance by vertebrate [Ca.sub.V] channels.