Annexins putative linkers in dynamic membrane–cytoskeleton

Summary.The plasma membrane,the most external cellular structure,is at the forefront between the plant cell and its environment. Hence,it isnaturally adapted to function in detection of external signals,their trans-duction throughout the cell,and finally,in cell reactions. Membranelipids and the cytoskeleton,once regarded as simple and static structures,have recently been recognized as significant players in signal transduc-tion. Proteins involved in signal detection and transduction are organisedin specific domains at the plasma membrane. Their aggregation allows tobring together and orient the downstream and upstream members of sig-nalling pathways. The cortical cytoskeleton provides a structural frame-work for rapid signal transduction from the cell periphery into thenucleus. It leads to intracellular reorganisation and wide-scale modula-tion of cellular metabolism which results in accumulation of newly syn-thesised proteins and/or secondary metabolites which,in turn,have to bedistributed to the appropriate cell compartments. And again,in plantcells,the secretory vesicles that govern polar cellular transport are deliv-ered to their target membranes by interaction with actin microfilaments.In search for factors that could govern subsequent steps of the cell re-sponse delineated above we focused on an evolutionary conserved pro-tein family,the annexins,that bind in a calcium-dependent manner tomembrane phospholipids. Annexins were proposed to regulate dynamicchanges in membrane architecture and to organise the interface betweensecretory vesicles and the membrane. Certain proteins from this familywere also identified as actin binding,making them ideal mediators incell membrane and cytoskeleton interactions.

Keywords:Plant annexin; Actin microfilament; Stress response; An-nexin–actin interaction; Exocytosis.

Annexins in plant cellsAnnexins constitute a family of ubiquitous,calcium- andmembrane-binding proteins. They have been intensivelystudied since the identification of the first annexin in animaltissues (Creutz etal. 1978) and subsequent recognition of

proteins with similar characteristics in plant cells (Bousteadetal. 1989,Blackbourn etal. 1991). In vertebrates the annex-ins were grouped into 13 families. In contrast,plant annexinsseem to represent a relatively simpler,smaller,and lessdiverse family of proteins. Nevertheless,in all analysedplant species,at least two distinct proteins with molecularmasses between 33 and 36kDa have been discovered(Smallwood etal. 1990,Shin etal. 1995,Proust etal. 1996,Thonat etal. 1997). However,a search through theArabidopsis thalianagenome,in which seven annexingenes were found (Clark etal. 2001),showed that thisnumber can be even larger. Summarising,it seems to betrue for both vertebrates and plants that more than one anne-xin is usually expressed at a given moment and in a par-ticular cell type (so-called annexin fingerprint). Thisindicates that in spite of significant functional homology,individual annexins support distinct and divergent func-tions. Some data suggest that individual annexins are asso-ciated with different cellular compartments possiblyconferring specificity of cellular response to the given stim-ulus. A special family of plant vacuolar annexins,VCaBP,with a slightly larger molecular mass (ca. 42kDa) wasdiscovered in various plant species of the familiesSolanaceaeand Brassicaceae (Seals etal. 1994,Seals andRandall 1997). In mustard plants (Sinapis alba),annexinp28 was shown to be a part of the chloroplast translationapparatus (Pfannschmidt etal. 2000). Finally,the presenceof nuclear annexins has also been documented (Clark etal.1998,Kovacs etal. 1998).Annexins have an evolutionary conserved overall struc-ture,with an about 70-amino-acid motif repeated fourtimes within the molecule,and contain a discrete (neitherEF-hand nor C2) calcium binding site (Fig.1). Calcium

Annexins:putative linkers in dynamic membrane–cytoskeleton interactions in plant cells

D. Konopka-PostupolskaLaboratory of Plant Pathogenesis,Institute of Biochemistry and Biophysics,Polish Academy of Sciences,WarsawReceived January 10,2006; accepted March 14,2006; published online April 24,2007©Springer-Verlag 2007

*Correspondence and reprints:Laboratory of Plant Pathogenesis,Insti-tute of Biochemistry and Biophysics,Polish Academy of Sciences,Pawinskiego 5A,02-106 Warsaw,Poland.

Protoplasma (2007) 230:203–215DOI 10.1007/s00709-006-0234-7PROTOPLASMA