A ten-residue domain (Y 11–A 20) in the NH 2-terminus modulates membrane association of annexin A7

Abstract

Annexin A7 (synexin, annexin VII) is postulated to promote membrane fusion during surfactant secretion in alveolar type II cells and catecholamine secretion in adrenal chromaffin cells. Recently, we demonstrated that the 1–29 residues in the NH 2-terminus could, possibly by interaction with the COOH-terminus, influence the Ca 2+-dependent membrane binding, aggregation, and fusion properties of annexin A7 (A7). In this study, we further investigated this 29-residue domain by evaluating several deletion and point mutations for membrane-associated functions of A7. In comparison to A7, the mutants lacking 1–29 residues (A7Δ 1–29) or 1–21 residues (A7Δ 1–21), but not those lacking 1–10 residues (A7Δ 1–10) or 21–29 residues (A7Δ 21–29), showed diminished membrane binding. Segmental deletion of 10–20 residues (A7Δ 10–20) also decreased the protein binding to membranes. The Ca 2+-dependent membrane aggregation of PLV with A7Δ 1–29 was maximally diminished but less so with A7Δ 10–20 or A7Δ 1–21 in comparison to that with A7. However, phospholipid vesicle (PVL) aggregation was unaffected with A7Δ 1–10 or A7Δ 21–29. The Ca 2+-dependent membrane fusion of PLV was also diminished with A7Δ 10–20 and A7Δ 1–29, but not with A7Δ 1–10. Since the mode of annexin A7 association and function with biological membranes could be different, we also evaluated these proteins for functional changes with isolated lung lamellar bodies. In comparison to A7, the binding to lamellar bodies was diminished for A7Δ 1–29 and A7Δ 1–21 but not for A7Δ 1–10. The Ca 2+-dependent fusion of isolated lamellar bodies with PLV was also diminished with A7Δ 1–29, but not with A7Δ 10–20 or A7Δ 1–21. Taken together, our studies suggest that the 10-residue domain (Y 11–A 20) in the NH 2-terminus modifies the phospholipid binding and aggregation properties of annexin A7. For binding and fusion of biological membranes, the 10–29-residue domain may be required although the annexin A7 properties are primarily modulated through the Y 11–A 20 domain.