CAN ENZYMATIC-ACTIVITY, OR OTHERWISE, BE INFERRED FROM STRUCTURAL STUDIES OF ANNEXIN-III

Annexin III, a putative inositol (1,2)-phosphohydrolase, was co-crysta llized with inositol 2-phosphate, the inhibitor of the reaction, and i ts structure was solved to 1.95 Angstrom resolution. No enzyme active site was observed in the structure, Assays for enzymatic activity were also negative. Search for annexin III-inositol phosphate interactions using the BIAcore(TM) system revealed an affinity for inositol cyclic (1,2)-phosphate, suggesting annexin III may sequester the molecule in the cell. The BIAcore(TM) system used with different phospholipids sh owed that annexin III displays specificity for phosphatidylethanolamin e, but not for phosphatidylinositols. Interestingly, a molecule of eth anolamine was found bound to the protein in the crystal structure. Cou pled with the fact that this is a particularly abundant phospholipid i n granules specific to neutrophils, cells where annexin III is highly expressed, our finding could be pointing to a physiological role of an nexin III.