A ten-residue domain (Y11–A20) in the NH2-terminus modulates membrane association of annexin A7

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 NH2-terminus could, possibly by interaction with the COOH-terminus, influence the Ca2+-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 (A7Δ1–29) or 1–21 residues (A7Δ1–21), but not those lacking 1–10 residues (A7Δ1–10) or 21–29 residues (A7Δ21–29), showed diminished membrane binding. Segmental deletion of 10–20 residues (A7Δ10–20) also decreased the protein binding to membranes. The Ca2+-dependent membrane aggregation of PLV with A7Δ1–29 was maximally diminished but less so with A7Δ10–20 or A7Δ1–21 in comparison to that with A7. However, phospholipid vesicle (PVL) aggregation was unaffected with A7Δ1–10 or A7Δ21–29. The Ca2+-dependent membrane fusion of PLV was also diminished with A7Δ10–20 and A7Δ1–29, but not with A7Δ1–10. 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 A7Δ1–29 and A7Δ1–21 but not for A7Δ1–10. The Ca2+-dependent fusion of isolated lamellar bodies with PLV was also diminished with A7Δ1–29, but not with A7Δ10–20 or A7Δ1–21. Taken together, our studies suggest that the 10-residue domain (Y11–A20) in the NH2-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 Y11–A20 domain.