Annexins of Plant Cells

Abstract

Ca2 plays a major role in stimulus-response coupling for many plant cell signaling pathways (Bush, 1995). A variety of stimuli, including light, touch, and orientation changes, can induce a sharp increase in the equilibrium level of free Ca2 in the cytoplasm of plant cells, from a concentration that is usually maintained at below 0.3 gM to often well above 1 ,tM (Trewavas and Gilroy, 1991). The incoming Ca2+ enters the cytoplasm from the extracellular space through channels in the plasma membrane or from internal stores such as the ER or vacuole through channels that are typically induced to open by inositol 1,4,5-trisphosphate (Ward et al., 1995). This increase in the free Ca21 concentration has profound effects on cellular metabolism, exerting its effects primarily by activating Ca2'-binding regulatory proteins. The identity and function of these stimulus-transducing proteins have been the subjects of considerable research activity in recent years. Calmodulin was the first discovered and is the best studied, but a number of other Ca2+binding proteins have been identified in plants. Prominent among these are the Ca2'-dependent protein kinases, whose properties have been previously reviewed (Roberts and Harmon, 1992). A relatively new group of Ca2+-binding proteins characterized in both plants and animals is the annexins. Their discovery is further evidence of the variation that exists in Ca2+ transduction pathways in plants. The annexins are a family of at least 13 structurally related proteins that show Ca2'-dependent phospholipid binding. In mammalian cells there is sequence evidence for 10 different annexins, and other distinct annexins have been found in Drosophila and Hydra (Johnston et al., 1990; Schlaepfer et al., 1992). These proteins have been intensely studied in animal cells. Although they were initially discovered as minor contaminants of calmodulin purifications in animal cells, annexins are distinct from the calmodulin family of Ca2+-binding proteins. Structurally, they are made up of four to eight repeats of 70 to 75 amino acids that contain a smaller, more highly conserved consensus sequence and do not contain an E-F-hand Ca2+-binding site. Annexins generally have a lower affinity for Ca21 than does calmodulin, and they require the presence of acidic phospholipids to exhibit binding at physiological Ca2+ concentrations. Their ability to annex or aggregate membranes at certain Ca2+ concentrations led to the initial interest in these proteins as components in Ca2'-mediated secretion. It is widely accepted that the reversible, Ca2+dependent binding of annexins to phospholipid bilayers is a peripheral interaction, although some effects of annexins on membranes suggest that they associate with them in a manner similar to transmembrane channel proteins (Swairjo and Seaton, 1994). The annexins carry out numerous functions in various cell types. The expression of individual members of the annexin family is differentially and developmentally regulated in mammalian systems. The annexin genes have maintained a moderate level of structural conservation while also accommodating changes in their primary sequences that confer different properties and functions. The N-terminal region of each annexin can vary in length and sequence, and this variation is an important basis of functional differences. It is within this region that regulatory posttranslational modifications such as phosphorylation and proteolysis are made and that cellular interactions with other proteins occur (Raynal and Pollard, 1994). The specific function of individual annexins is still unclear. It is evident, however, that they are multifunctional and play a role in a wide variety of essential cellular processes, such as membrane trafficking, membrane channel activity, phospholipid metabolism, mitotic signaling, and DNA replication (Raynal and Pollard, 1994). In the past several years the results of x-ray crystallography, electrophysiological analysis, and homology-modeling studies have shown that certain annexins may function directly as Ca2+ channels. Specifically, animal annexins I, V, VI, and VII all exhibit in vitro voltage-gated Ca2+ channel activity, as assayed in model membranes by patch-clamping techniques (Pollard et al., 1992). X-ray crystallography and homology modeling have been used to confirm that these proteins have a predicted hydrophilic channel (Huber et al., 1992; Chen et al., 1993). All of the postulated functions of annexins were first discovered in animal cells, and all of the early work on the structure and biochemistry of annexins was done on animal annexins (reviewed by Dedman, 1986; Burgoyne and Geisow, 1989; Romisch and Paques, 1991; Raynal and Pollard, 1994; Swairjo and Seaton, 1994). Only during the past 5 to 6 years has research on plant annexins begun to provide insights into the importance of these proteins in plant growth and development. In this review we will summarize and critique available data on the annexins in plants. ' Work in the authors' laboratory on annexins is supported by the National Aeronautics and Space Administration. * Corresponding author; e-mail sroux@uts.cc.utexas.edu; fax 1-512-471-3878.