Biophysical and molecular properties of annexin-formed channels

Abstract

The annexins are water soluble proteins possessing a hydrophilic surface, which belong to a family of proteins which (a) bind ('annex') both calcium and phospholipids, and (b) form voltage-dependent calcium channels within planar lipid bilayers. Annexins types are diverse (94 annexins in 45 species) and they belong to an enormous multigene family that ranges throughout all eukaryotic kingdoms. Although the structure of these proteins is now well known their functional and physiological roles remain largely unknown and circumstantial. Various experimental approaches provided evidence that annexins function as Ca(2+) channels that could act as regulators of membrane fusion. The identity of annexins is derived from the conserved 34 kDa C-terminal domain which comprises four repeats - except for annexin VI, with eight repeats - of a sequence of approximately seventy amino acids, which holds the area known as the 'endonexin fold', with its identifying GXGTDE. Annexins have been placed into three subgroups of (1) tetrad core and short amino terminal, (2) tetrad core and long amino terminal, and (3) octad core and short amino terminal. The repeats are highly conserved, each forming a compact alpha-helical domain comprising five alpha-helices wound in a right-handed superhelix. Four domains are formed, arranged in a nearly flat and cyclical array, with domains I and IV, and II and III respectively forming two tightly organised modules with almost twofold symmetry. A hydrophilic pore lies at the centre of the molecule, forming a prominent ion channel coated with charged and highly conserved residues. The annexin molecule is slightly curved, with both a convex and a concave face. The cation/anion permeability ratios and the selectivity sequence of the ion channels formed by several annexins confirm the selectivity of the annexins for Ca(2+) over other divalent cations, and reveals the importance of structural sites, e.g. amino acid positions 17, 78, 95 and 112 for the identification of the ion channel's position, function and regulation. Some are sensitive to low doses of the phenothiazine drugs, trifluoperazine (an anti-schizophrenia drug) and promethazine (anti nausea drug) La(3+) and Cd(2+), (blockers of voltage-gated Ca(2+) channels) nifedipine (an inhibitor of non-activating Ca(2+) channels). There are two main competing models used to explain in vitro ion channel activity of annexins: one involves changes in the conductance of ion via electrostatic disturbance of the membrane surface; the other involves a much more extensive alteration in protein structure and a correspondingly deeper penetration into the membrane.

Serum ferritin and transferrin levels as serologic markers of methylene diphenyl diisocyanate–induced occupational asthma

“…41,42 Finally, multiple annexins were downregulated in the MDI-OA group. Annexins are watersoluble proteins that form voltage-dependent calcium channels within planar lipid bilayers, 43 and they are expressed at reduced levels in asthmatic mice. 42 Additional studies are needed to evaluate the clinical relevance of each protein for MDI-exposed workers.…”

Annexins: Components of the Calcium and Reactive Oxygen Signaling Network

“…In animals, annexin A5 is one of several annexins that can form a Ca 2+ -permeable channel in nonoxidized membranes but is also the likely candidate for mediating peroxide-induced Ca 2+ influx across the PM of chicken DT40 cells (Kubista et al, 1999;Kourie and Wood, 2000;Gerke and Moss, 2002). Unlike conventional channels, annexins are soluble phospholipidbinding proteins that can undergo conditional association with or insertion into membranes, directly from the soluble phase (Kourie and Wood, 2000;Gerke and Moss, 2002;Gorecka et al, 2007;Mortimer et al, 2008;Laohavisit and Davies, 2009).…”

“…Unlike conventional channels, annexins are soluble phospholipidbinding proteins that can undergo conditional association with or insertion into membranes, directly from the soluble phase (Kourie and Wood, 2000;Gerke and Moss, 2002;Gorecka et al, 2007;Mortimer et al, 2008;Laohavisit and Davies, 2009). The presence of a hydrophilic pore at the center of the molecule is proposed to be the structural basis for annexin Ca 2+ channel activity (Gerke and Moss, 2002).…”

Arabidopsis Annexin1 Mediates the Radical-Activated Plasma Membrane Ca2+- and K+-Permeable Conductance in Root Cells

“…Annexins are membrane binding proteins found in pro-and eukaryotes (Morgan et al, 2006). Several animal annexins have been reported to function in vitro as Ca 2+ channels, including vertebrate annexins A1, 2, 5 to 7, and 12 (Burger et al, 1994;Liemann et al, 1996;Kourie and Wood, 2000). Loss-of-function mutants may have impaired ability to regulate [Ca 2+ ] cyt , for example, A5 (2/2) chicken DT40 cells (Kubista et al, 1999), A7 (+/2) murine brain cells (Watson et al, 2004), and A7 (2/2) murine cardiomyocytes (Schrickel et al, 2007).…”

“…However, no studies have causally linked in vitro annexin transport activity to that in native membrane (KonopkaPostupolska et al, 2011). Annexins are abundant throughout the plant kingdom (reviewed in Laohavisit and Davies, 2011a), and some contain the charged residues found in the hydrophobic pore of channel-forming animal annexins (Kourie and Wood, 2000;Laohavisit et al, 2009;Laohavisit and Davies, 2011a). Of these, Capsicum annuum annexin32/24 (ANN32/24) mediates Ca 2+ influx in liposomes (Hofmann et al, 2000), while maize annexins ANN33 and ANN35 support a Ca 2+ -permeable conductance in planar lipid bilayers (PLBs) (Laohavisit et al, 2009(Laohavisit et al, , 2010, and Arabidopsis ANN1 forms a K + -permeable channel in PLB (Gorecka et al, 2007).…”