1.A.31 The Annexin (Annexin) Family
The annexins are a structurally conserved family of proteins characterized by reversible Ca2+-dependent intracellular membrane/phospholipid binding. Membrane association is critical for their proposed functions which include vesicle trafficking, membrane repair, membrane fusion and ion channel formation (McNeil et al., 2006). High-resolution crystal structures of the soluble forms of several annexins are available. These include hydra annexin XII and human annexin V. A low resolution structure is available for the membrane-bound Annexin 5 trimer (Oling et al., 2000). These proteins bind to surfaces of phosphatidylserine-containing phospholipid bilayers either in the presence of Ca2+ or under conditions of low pH (pH 5-6). Then they undergo major conformational changes involving three states: (1) soluble state (monomer) → (2) peripheral membrane-associated state (trimer) → (3) integral transmembrane channel state (hexamer). This last state requires major conformational changes with the formation of a putative polytopic, amphipathic channel. Ca2+ induces dimer, trimer and hexamer formation as well as phospholipid association. A helix-loop-helix structure in the soluble form is believed to be converted into one of the continuous transmembrane α-helices.
All annexins display a conserved core domain consisting of four homologous repeats, each of about 70 residues. Two of these repeat units may comprise a single Ca2+/phospholipid binding site. Some annexins (e.g., Annexin VI) are twice as large as others (e.g., Annexin X) because of an intragenic duplication. The ion channel properties of the integral membrane forms of annexins have been amply documented. However, it is not clear that channel activity explains all of their biological properties. For example, Annexin VI has been reported to modulate maxichloride channel currents as well as K+ and Ca2+ currents in different cell types, possibly by regulating the activities of other channels (Riquelme et al., 2004). Annexins are also called Lipocortins, Synexins, Endonexins and Calpactins. Most are 310-350 residues long.
Annexins comprise a large family derived from animals with many paralogues in any one. They are found throughout the eukaryotic kingdoms. They have been subdivided into three groups: (1) tetradcore with short amino termini; (2) tetradcore with long amino termini; (3) octadcore with short amino termini. The core is a 34 kDa C-terminal domain of 4 repeats except for annexin VI which has 8 repeats. Each repeat is 70aas with an 'endonexin fold' with its identifying GXGTDE sequence. Each repeat forms a compact α-helical domain consisting of 5 α-helicies wound in a right-handed superhelix. The four domains are arranged in a flat cylindrical array with the hydrophilic channel in the center of the molecule. Ca2+ is preferred over other divalent cations, but both cations and anions can be transported.
Medicago truncatula annexin 1 (AnnMt1) participates in nodulation (Nod factor signaling) and mycorrhization in legume plants. AnnMt1 mediates non-selective membrane permeabilization to cations with conductances ranging from 16 pS to 329 pS (Kodavali et al. 2013). In agreement with other structurally determined annexins, homology modeling of AnnMt1 suggests that most of the functional determinants are on the convex surface of the protein.
Annexins are soluble proteins that bind acidic phospholipids such as phosphatidylserine in a calcium-dependent manner. Observations of specific ion conductances in annexin-bound membranes is controversial (Reviakine 2018). The controversy considering the well-characterized peripheral assembly of the annexins at membranes vs. their transmembrane insertion and mediation of lipid rearrangements is the subject of a review (Reviakine 2018).
At least one transport reaction catalyzed by annexin channels is:
Ions (in) ions (out)