Annexins as Overlooked Regulators of Membrane Trafficking in Plant Cells

Abstract

Annexins are an evolutionary conserved superfamily of proteins able to bind membrane phospholipids in a calcium-dependent manner. Their physiological roles are still being intensively examined and it seems that, despite their general structural similarity, individual proteins are specialized toward specific functions. However, due to their general ability to coordinate membranes in a calcium-sensitive fashion they are thought to participate in membrane flow. In this review, we present a summary of the current understanding of cellular transport in plant cells and consider the possible roles of annexins in different stages of vesicular transport.

Keywords: Rab protein; SNARE; annexin; membrane trafficking; stress response; vesicular transport.

Figure 1

Predicted structure of three Arabidopsis annexins and proposed mechanism for annexin-membrane coordination. (A) Predicted structure of three Arabidopsis annexins, ANNAT1, ANNAT3, and ANNAT4. The structure was prepared with Swiss-PdbViewer, DeepView v4.1 by Nicolas Guex, Alexandre Diemand, Manuel C. Peitsch, and Torsten Schwede on the basis of existing annexin crystal structures. The overall structure of annexins is evolutionary conserved. The molecule consists of four repeats (I–IV) of approximately 70 amino acids (PFAM domain PF00191, 66 aa). In plant annexins the type II Ca²⁺- and phospholipids binding motif (GxGT-(38–40 residues)-D/E) is highly conserved in repeat I (in grey), generally lost in repeats II and III, and only moderately conserved in repeat IV (in red). In Arabidopsis, the canonical motif is present in repeat 1 of annexin 1 and 3 and a modified motif in repat IV of annexin 1 and 3. In annexin 4 there is no recognizable calcium and phospholipids binding motifs; (B) Possible mechanism of membrane coordination by annexins, according to [34,37]. Two opposing membranes can be coordinated by dimerizing annexin molecules. Binding to the membrane causes changes in molecular conformation and flattening of protein disc. As a result, a secondary calcium- and membrane-binding sites on the concave surface disclose, which allows positioning of the various membrane structures.

Figure 2

Model depicting intracellular transport in plant cells.

Figure 3

Potential targets for annexin participation in intracellular transport in plant cells. There is evidence that annexins participate in both the early and late secretory pathways including endo- and exocytosis. Annexins are also suggested to function in conventional constitutive secretion as well as non-conventional secretion; green arrow—forward route, red arrow—reverse route; yellow arrow—endocytic route.