IDENTIFICATION AND CHARACTERIZATION OF A NOVEL TYPE OF ANNEXIN-MEMBRANE INTERACTION - CA2-II WITH EARLY ENDOSOMES( IS NOT REQUIRED FOR THE ASSOCIATION OF ANNEXIN)

Annexin II, a member of a family of Ca2+ and membrane binding proteins , has been implicated in regulating membrane organization and membrane transport during endocytosis and Ca2+ regulated secretion. To charact erize the mechanistic aspects of the annexin II action we studied para meters which determine the endosomal association of annexin II. Immuno blot analysis of subcellular membrane fractions prepared from BHK cell s in the presence of a Ca2+ chelating agent reveals that annexin II re mains associated with endosomal membranes under such conditions. This annexin II behaviour is atypical for the Ca2+ regulated annexins and i s corroborated by the finding that ectopically expressed annexin II mu tants with inactivated Ca2+ binding sites continue to co-fractionate w ith endosomal membranes, The Ca2+-independent membrane association of annexin TT is also not affected by introducing mutations interfering w ith the complex formation of annexin II with its intracellular protein ligand p11. However, a deletion of the unique N-terminal domain of an nexin II, in particular the sequence spanning residues 15 to 24, aboli shes the Ca2+-independent association of the protein with endosomes. T hese results describe a novel, Ca2+-independent type of annexin-membra ne interaction and provide a first explanation for the observed prefer ence of different annexins for different cellular membranes, In the ca se of annexin II this specificity could be mediated through specific m embrane receptors interacting with a unique sequence in the annexin II molecule.