title: Calcium-activated endonexin II forms calcium channels across acidic phospholipid bilayer membranes.
abstract: 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.
authors.affiliation: Laboratory of Cell Biology and Genetics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: E
authors.initials: E
authors.lastname: Rojas
authors.forename: H B
authors.initials: HB
authors.lastname: Pollard
authors.forename: H T
authors.initials: HT
authors.lastname: Haigler
authors.forename: C
authors.initials: C
authors.lastname: Parra
authors.forename: A L
authors.initials: AL
authors.lastname: Burns
mesh_terms: D000595:Amino Acid Sequence; D017304:Annexin A5; D002118:Calcium / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism* / Q000502:physiology; D002135:Calcium-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008051:Lipid Bilayers; D008433:Mathematics; D008564:Membrane Potentials; D008958:Models, Molecular; D008962:Models, Theoretical; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D017304:Annexin A5; D015220:Calcium Channels; D002135:Calcium-Binding Proteins; D008051:Lipid Bilayers; D011994:Recombinant Proteins; D002118:Calcium