Modulation by Ca2+ and by membrane binding of the dynamics of domain III of annexin 2 (p36) and the annexin 2-p11 complex (p90): Implications for their biochemical properties

The modulation of the local structure and dynamics of domain III of annexin 2 (Anx2), in both the monomeric (p36) and heterotetrameric forms (p90), by calcium and by membrane binding was studied by time-resolved fluorescence intensity and anisotropy measurements of the single tryptophan residue (W21 2). The results yield the same dominant excited-state lifetime (1.4 ns) in both p36 and p90, suggesting that the conformation and environment of W212 are very similar. The fluorescence anisotropy decay data were analyzed by a ssociative (two-dimensional) as well as nonassociative (one-dimensional) mo dels. Although no statistical criterion is decisive for one model versus th e other, only the associative model allows recovery of a physically relevan t value of the Brownian rotational correlation of the protein. Using the as sociative model, a nanosecond flexibility is detectable in p90 but not in p 36. When Ca2+ binds in the millimolar concentration range to both forms of Anx2 a conformational change takes place leading to an increase of the majo r excited-state lifetime (2.6 ns) and to a suppression of the W212 local fl exibility of p90. Binding to membranes of either p36 or p90 in the presence of Ca2+ does not induce any conformational change other than that provoked by Ca2+ binding alone. The W212 local flexibility in both proteins increas es significantly, however, in their membrane-bound forms. In the presence o f membranes, the conformation change of domain III in p90 displays a sensit ivity to Ca2+ 2 orders of magnitude higher than that of p36, reaching intra cellular sub-micromolar concentration ranges. This higher Ca2+ sensitivity correlates with the Ca2+-dependent membrane aggregation but not with their Ca2+-dependent binding to membranes. The significance of these structural a nd dynamical changes for the function of the protein is discussed.