SIMILARITY IN CALCIUM-CHANNEL ACTIVITY OF ANNEXIN-V AND MATRIX VESICLES IN PLANAR LIPID BILAYERS

Matrix vesicles (MVs), structures that accumulate Ca2+ during the init iation of mineral formation in growing bone, are rich in annexin V. Wh en MVs are fused with planar phospholipid bilayers, a multiconductance Ca2+ channel is formed, with activity essentially identical to that o bserved when annexin V is delivered to the bilayer with phosphatidylse rine liposomes. Ca2+ currents through this channel, from either MV or annexin V liposomes, are blocked by Zn2+, as is Ca2+ uptake by MV incu bated in synthetic cartilage lymph. Blockage by Zn2+ was most effectiv e when applied to the side containing the MV or liposomes. ATP and GTP differentially modulated the activity of this channel: ATP increased the amplitude of the current and the number of conductance states; GTP dramatically reduced the number of events and conductance states, lea ding to well-defined Ca2+ channel activity from either MV or the annex in V liposomes. In the distinctive effects of ATP, GTP, and Zn2+ on th e Ca2+ channel activity observed in both the MV and the liposome syste ms, the common factor was the presence of annexin V. From this we conc lude that Ca2+ entry into MV results from the presence of annexin V in these membrane-enclosed structures.