Calcium-activated endonexin II forms calcium channels across acidic phospholipid bilayer membranes

Human endonexin II (annexin V) and recombinant human endonexin II can be activated by Ca²⁺ to interact with acidic phospholipid bilayers formed at the tip of a patch pipette. Once associated with the bilayer, endonexin II forms voltage-gated channels which are selective for divalent cations according to the following series Ca²⁺ > Ba²⁺ > Sr²⁺ ≫ Mg²⁺. However, endonexin II also expresses a selective affinity for Ca²⁺ which is manifest by an observed reduced current through the open channel when Ca²⁺ is the charge carrier. La³⁺ blocks endonexin II channels, as it does synexin (annexin VII) and other types of Ca²⁺ channels. However, as with synexin, the dihydropyridine Ca²⁺ channel antagonist nifedipine does not affect endonexin II channel activity. Endonexin II channels are also permeant to Li⁺, Cs⁺, Na⁺, and to a lesser extent, K⁺, resembling in this manner Ca²⁺ release channels from sarcoplasmic reticulum. Indeed, the low affinity of endonexin II channels for such ions as Cs⁺ or Li⁺ have allowed us to use these cations for measurement of the kinetic properties of the channel, with minimal concerns for the ion/channel interactions observed with the physiological substrate, Ca⁺. Finally, we observed that endonexin II channel activity always occurred in bursts, making necessary the use of two exponential functions to fit open- and closed-time histograms. We conclude from these data that the domain responsible for endonexin II channel activity, first observed by ourselves in the homologue synexin, is probably the C-terminal tetrad repeat common to both molecules.