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+ greater than Ba2+ greater than Sr2+ much greater than 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.[1]References
- Calcium-activated endonexin II forms calcium channels across acidic phospholipid bilayer membranes. Rojas, E., Pollard, H.B., Haigler, H.T., Parra, C., Burns, A.L. J. Biol. Chem. (1990) [Pubmed]
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