Annexins-a family of proteins with distinctive tastes for cell signaling and membrane dynamics

Abstract

Annexins are cytosolic proteins with conserved three-dimensional structures that bind acidic phospholipids in cellular membranes at elevated Ca²⁺ levels. Through this they act as Ca²⁺-regulated membrane binding modules that organize membrane lipids, facilitating cellular membrane transport but also displaying extracellular activities. Recent discoveries highlight annexins as sensors and regulators of cellular and organismal stress, controlling inflammatory reactions in mammals, environmental stress in plants, and cellular responses to plasma membrane rupture. Here, we describe the role of annexins as Ca²⁺-regulated membrane binding modules that sense and respond to cellular stress and share our view on future research directions in the field.

Fig. 1. Annexin structural organization.

Illustrated (top) is the 3D structure of annexin 5 (Protein Data Bank IA8A; Swairjo et al.). It is used here as an example of the canonical annexin fold. Green circles (size exaggerated) indicate the location of bound Ca²⁺ ions. The annexin ligand indicated by an arrow is a glycerophosphoserine molecule—here representing the membrane lipid phosphatidylserine. The sequence data and some amino acid annotations below the 3D structure have been taken from the UniProt database. This currently contains over 30,000 annexin sequences, but selected regions of human (h), mouse (m), and pig (p) AnxA1 and two plant kingdom annexins from (1) cotton (cAnxD) and (2) a bell pepper (pAnxD) are included for comparison as evolutionary distant proteins. The sequences have initially been aligned by the CLUSTAL program. Selected homologous and/or known invariant residues are shown in bold font. Key serine (S) and tyrosine (Y) phosphorylation sites are shown in blue. From inspection of the Protein Structure Data Base (PDB), residues in the unstructured amino-terminal domains have been highlighted in magenta and those within the secondary structures at the commencement of the annexin core domain (helices 1–2 and part of helix 3) are highlighted in yellow. The corresponding regions of the annexin 5 structure shown at the top of the figure have been analogously colored. There are no experimental 3D structures for AnxA7, 9, 10, or 11 and so helices of the other annexins have not been highlighted in the illustration, but can be found in the PDB or can be inferred from strong sequence homologies. To accommodate the alignments in an accessible figure, the long amino termini of hAnx7 and 11 have been truncated. Similarly, the sequences within the annexin core domain have been arbitrarily interrupted within the third helix. Were the complete core domain sequences to be aligned in this manner, this would demonstrate analogous alignments of the 4 or (uniquely 8 for AnxA6) sequence repeats of ~70 residues which constitute the cores of all annexins.

Fig. 2. Assorted annexin functions at membrane interfaces.

Examples are shown for AnxA1, AnxA2, and AnxA11. AnxA2 (green) in a heterotetrameric complex with S100A10 (purple) can coordinate plasma membrane domains that are enriched in negatively charged phospholipids such as phosphatidylserine (PS) and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) and also cholesterol and are linked to cortical F-actin (1). The AnxA2-S100A10 heterotetramer also supports the fusion of exocytic vesicles by linking PI(4,5)P₂-rich domains to tethering factors (2) and it interacts with ion channels (blue) and GPCRs (rose), positioning and anchoring them at the plasma membrane (3). AnxA1 (turquois) is a substrate of the activated growth factor receptor EGFR (orange), and the heterotetrameric complex consisting of AnxA1 and S100A11 (ocher) is involved in the sorting of EGFR onto luminal vesicles of multivesicular endosomes (4, 5) and the establishment of ER–endosome membrane contact sites. AnxA11 (gray) tethers membraneless RNA granules to mobile lysosomes (6). See text for references. Generated with BioRender.com.

Fig. 3. Annexins in plasma membrane repair.

Ca²⁺ influx following plasma membrane injury triggers the recruitment of several annexins to the wound site. Working in concert, annexins such as AnxA4 then generate forces to curve the free edges of the injured membrane, while AnxA6 produces constriction forces along free membrane edges to eventually support the resealing (1). This is facilitated by the Ca²⁺-triggered exocytosis of early endosomes and lysosomes that likely provides additional membrane (2)^–. Only AnxA4 and AnxA6 are shown here for simplicity. Other annexins recruited to the wound site and participating in repair are AnxA1 and AnxA2 which most likely support fusion, AnxA5 which provides a protein lattice inhibiting wound expansion, and AnxA7 which establishes a link to the ESCRT machinery (3, see the text for references). Generated with BioRender.com.

Fig. 4. Plant annexins, stress response and global warming.

Graphic showing a selection of the putative functions of plant annexins highlighted in the different petals. The relevant annexin gene names shown are: AtANN1x (Arabidopsis thaliana—Thale cress), STANN1 (Solanum tuberosum—potato) and TaANN2 & 12 (Triticum aestivum—common wheat). As plant annexins play an important role in adaptation to drought and salt stress, conditions that will be increasingly met in future climates, the petals are superimposed on a bar chart indicating global land temperature change (plotted as a difference from average between 1880 and 2020; source: NOAA climate.gov; graph based on data from the National Centers for Environmental Information).

Fig. 5. From molecules to cells to systems—annexin research across scales.

Nanotechnological tools such as solid-supported or vesicular lipid bilayers are employed to assess molecular principles governing the interaction of annexins with membranes (1). Cell culture models in 2D and 3D are exploited to analyze annexin functions at the cellular level (2). These approaches together with work in whole organisms will result in a systemic understanding of annexin function leading to the exploitation of annexin properties in the diagnostics and possibly the treatment of human pathologies (3) (see the text for details). Green spheres represent annexins and red lightning flashes should highlight stress that impacts membrane integrity. Generated with BioRender.com.