Protoplasma (2007) 230: 203–215 DOI 10.1007/s00709-006-0234-7 PROTOPLASMA Printed in Austria Annexins: putative linkers in dynamic membrane–cytoskeleton interactions in plant cells D. Konopka-Postupolska Laboratory of Plant Pathogenesis, Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw Received January 10, 2006; accepted March 14, 2006; published online April 24, 2007 © Springer-Verlag 2007 Summary. The plasma membrane, the most external cellular structure, proteins with similar characteristics in plant cells (Boustead is at the forefront between the plant cell and its environment. Hence, it is et al. 1989, Blackbourn et al. 1991). In vertebrates the annex- naturally adapted to function in detection of external signals, their trans- duction throughout the cell, and finally, in cell reactions. Membrane ins were grouped into 13 families. In contrast, plant annexins lipids and the cytoskeleton, once regarded as simple and static structures, seem to represent a relatively simpler, smaller, and less have recently been recognized as significant players in signal transduc- diverse family of proteins. Nevertheless, in all analysed tion. Proteins involved in signal detection and transduction are organised in specific domains at the plasma membrane. Their aggregation allows to plant species, at least two distinct proteins with molecular bring together and orient the downstream and upstream members of sig- masses between 33 and 36 kDa have been discovered nalling pathways. The cortical cytoskeleton provides a structural frame- (Smallwood et al. 1990, Shin et al. 1995, Proust et al. 1996, work for rapid signal transduction from the cell periphery into the Thonat et al. 1997). However, a search through the nucleus. It leads to intracellular reorganisation and wide-scale modula- tion of cellular metabolism which results in accumulation of newly syn- Arabidopsis thaliana genome, in which seven annexin thesised proteins and/or secondary metabolites which, in turn, have to be genes were found (Clark et al. 2001), showed that this distributed to the appropriate cell compartments. And again, in plant number can be even larger. Summarising, it seems to be cells, the secretory vesicles that govern polar cellular transport are deliv- ered to their target membranes by interaction with actin microfilaments. true for both vertebrates and plants that more than one anne- In search for factors that could govern subsequent steps of the cell re- xin is usually expressed at a given moment and in a par- sponse delineated above we focused on an evolutionary conserved pro- ticular cell type (so-called annexin fingerprint). This tein family, the annexins, that bind in a calcium-dependent manner to membrane phospholipids. Annexins were proposed to regulate dynamic indicates that in spite of significant functional homology, changes in membrane architecture and to organise the interface between individual annexins support distinct and divergent func- secretory vesicles and the membrane. Certain proteins from this family tions. Some data suggest that individual annexins are asso- were also identified as actin binding, making them ideal mediators in ciated with different cellular compartments possibly cell membrane and cytoskeleton interactions. conferring specificity of cellular response to the given stim- Keywords: Plant annexin; Actin microfilament; Stress response; An- ulus. A special family of plant vacuolar annexins, VCaBP, nexin–actin interaction; Exocytosis. with a slightly larger molecular mass (ca. 42 kDa) was discovered in various plant species of the families Annexins in plant cells Solanaceae and Brassicaceae (Seals et al. 1994, Seals and Randall 1997). In mustard plants (Sinapis alba), annexin Annexins constitute a family of ubiquitous, calcium- and p28 was shown to be a part of the chloroplast translation membrane-binding proteins. They have been intensively apparatus (Pfannschmidt et al. 2000). Finally, the presence studied since the identification of the first annexin in animal of nuclear annexins has also been documented (Clark et al. tissues (Creutz et al. 1978) and subsequent recognition of 1998, Kovacs et al. 1998). Annexins have an evolutionary conserved overall struc- ture, with an about 70-amino-acid motif repeated four * Correspondence and reprints: Laboratory of Plant Pathogenesis, Insti- tute of Biochemistry and Biophysics, Polish Academy of Sciences, times within the molecule, and contain a discrete (neither Pawinskiego 5A, 02-106 Warsaw, Poland. EF-hand nor C2) calcium binding site (Fig. 1). Calcium 204 D. Konopka-Postupolska: Annexins in regulation of membrane–cytoskeleton dynamics D. Konopka-Postupolska: Annexins in regulation of membrane–cytoskeleton dynamics 205 binding induces structural changes resulting in protein regulation of membrane organisation, membrane traffick- translocation from the cytoplasm to the cell periphery. ing, interactions with the cytoskeleton, and secretion. In Crystallographic data reveal that membrane binding oc- time it became clear that certain plant annexins can also curs via formation of a ternary complex between annexin, function in plant stress response. Expression of different calcium, and the membranes (Swairjo et al. 1995). Annex- annexins was induced by osmotic stress (AnnMs2 from ins have been shown to preferentially bind to negatively alfa alfa [Kovacs et al. 1998]; AnnAt1 from A. thaliana charged membrane phospholipids (e.g., phosphatidylse- [S. Lee et al. 2004 and our unpubl. data]), which suggests rine). Plant annexins are fairly abundant cellular proteins that they might participate in drought resistance. Some (Clark and Roux 1995) and should thus be considered as a data indicate also that AnnAt1 can act at a crossroad be- very important element of calcium signalling pathways. tween auxin and abscisic acid signalling (Bianchi et al. On the basis of immunocytochemical experiments, it has 2002). Enhanced expression of annexin was also reported been concluded that in plant cells, annexins are localised after different treatments that led to accumulation of reac- mainly in the cytoplasm. When calcium levels increase, tive oxygen species during defence response in tomato they are moved towards the cytoplasmic surface of certain plants (Xiao et al. 2001), and after salicylic acid and hy- membrane structures, mainly the plasma membrane, but drogen peroxide treatment in Arabidopsis plants (Gidrol the presence of an annexin that binds specifically to the et al. 1996). It is worth mentioning that those two an- outer membrane of the chloroplast envelope was also re- nexins, namely, p34 from tomato cells and AnnAt1 from ported (Seigneurin-Barny et al. 2000). On the other hand, A. thaliana, represent close homologs (84% of homology experimental data show that particular proteins, although on protein level). AnnAt1 also has the ability to protect lacking defined targeting sequences, are present in non- heterologous cells from the consequences of oxidative stimulated cells in different cellular compartments. Proteo- stress. Molecular mechanisms of this protection have not mic analysis revealed that, e.g., AnnAt1 from A. thaliana been elucidated, although different hypotheses are consid- was present in the fraction of cell wall proteins (P. Woj- ered, beginning from an intrinsic peroxidase activity of taszek, Adam Mickiewicz University, Poznan, Poland, pers. AnnAt1 (Gidrol et al. 1996). An indirect effect via modu- commun.), integral membrane proteins (Santoni et al. 1998, lation of calcium signalling that results in the lowering of S. Lee et al. 2004), in central, vegetative vacuoles (Carter superoxide production and reduction of protein kinase C et al. 2004), as well as in the nuclear matrix (A. Jerz- activity was also proposed (Kush and Sabapathy 2001, manowski, Institute of Biochemistry and Biophysics, Polish Janicke et al. 1998). It is also possible that it can be a non- Academy of Sciences, Warsaw, Poland, pers. commun.). specific consequence of membrane lipid protection Mimosa annexin exhibits day–night changes in distribution: against oxidative stress. Mammalian annexin A5 was during the daytime it is localised on the cell periphery, shown to bind, with affinity similar to that of phos- while at night it stays in the cytoplasm (D. Hoshino et al. phatidylserine, to malondialdehyde adducts, a major prod- 2004). Additional analyses are necessary to establish uct of lipid peroxidation generated by the nonenzymatic whether this microcompartmentalisation is also true for the reaction of polyunsaturated fatty acids with molecular other plant annexins. oxygen (Balasubramanian et al. 2001). In contrast to free Despite several years of investigation, the primary radicals, lipid peroxides are long-lived and can thus dif- physiological function for annexins has not yet been elu- fuse from the site of origin and exert deleterious effects cidated. It is generally assumed that annexins are impli- and/or activate stress-related pathways in surrounding tis- cated in several processes related to membranes, including sues. If AnnAt1, and possibly other annexins, shares this Fig. 1. Alignment of the deduced amino acid sequences of human (A1, A2, A4, A5, A6, A7, A11, A13) and plant (Arabidopsis thaliana AnnAt1 to -7, Capsicum annuum Ca_p32 and p38, Lycopersicon esculentum Le_p34 and p35, Gossypium hirsutum Gh1, Nicotiana tabacum Nt_VCaBP, Solanum tuberosum St_p34, and Medicago sativa MsAnn annexin genes obtained using T-COFFEE (Notredame et al. 2000). Potential functional domains are indicated as follows: