Lamellar body fusion for surfactant secretion: Deletion of the 10–20 domain in the N‐terminus decreases fusion activity of annexin A7

Fusion between lamellar bodies and plasma membrane is obligatory for surfactant secretion. Our previous in vitro studies showed that annexin A7 (A7, synexin), which binds to membranes in a Ca 2+ ‐dependent manner, promotes such fusion, and increases surfactant PC secretion in permeabilized type II cells. All putative Ca 2+ and phospholipid (PL) binding sites are present in the C‐terminus and none is present in the 164‐residue N‐terminus of A7. However, deletion of the first 30 residues in the N‐terminus affected several properties of the C‐terminus, suggesting interactions between the two termini of A7. In this study, we characterized proteins with smaller deletions in the first 30 residues of A7. We deleted the first 29, 24, 21, or 10 residues (D29, D24, D21, or D10), or the domain between 10 and 20 (D10–20), or 21 and 29 (D21–29) residues. The Ca 2+ ‐dependent binding to PL vesicles (PLV) was lower for D29, D24, D21, or D10–20, but not for D10 or D21–29 in comparison to A7, suggesting the importance of 10–20 domain in protein binding to membranes. The binding of D29, D24, or D21, but not of D10, to lamellar bodies was also decreased in comparison to A7. The PLV aggregation was lower with D29, D21 and D10–20, but not with D10 or D21–29, in comparison to A7. The deletion of 21 or 29, but not of the first 10 residues also decreased the fusion of PLV or lamellar bodies. Thus, our studies suggest that the 10–20 domain in the N‐terminus can interact with the C‐terminus to modify Ca 2+ ‐dependent membrane binding and fusion activity of A7. We speculate that this domain is important in A7‐mediated membrane fusion during surfactant secretion.