Structural determinant of the vesicle aggregation activity of annexin I

Some annexins, including annexins I, II, IV, and VII, can promote membrane aggregation. To identify amino acids involved in annexin I-mediated membrane aggregation, we generated truncated mutants of human annexin I lacking various parts of the amino terminus. The in vitro vesicle binding and aggregation activities of these mutants indicated that both the amino-terminal region of annexin I spanning residues 26-29 and the carboxy-terminal core are involved in membrane aggregation. This notion was further supported by the finding that a chimera composed of residues 24-35 of annexin I and the core of annexin V has vesicle aggregation activity that is significantly higher than that of annexin V but lower than that of annexin I. Further site-specific mutations in the amino-terminal region of annexin I indicated that Lys-26 and Lys-29 are essential for its membrane aggregation activity. The comparison of tryptic digest patterns of free and vesicle-bound wild type and K29E mutant suggests that a primary role of Lys-26 and Lys-29 is to induce and stabilize an active conformation of annexin I for vesicle aggregation.