The exocyst complex in plants.

Abstract

Plant cell morphogenesis requires precise regulation of localized cell expansion and cell division. Vectorial exocytosis is a major morphogenetic process in plant cells, intimately interwoven with the dynamics of the cytoskeleton. Small GTPases of the Ras superfamily act as molecular switches participating in the control of both vesicle trafficking (Rab GTPases) and cytoskeleton dynamics (Rho GTPases). In tip-growing plant cells, such as the pollen tubes, wall formation is confined to the tip. The cytoplasm is highly polarized and grows by exocytosis in the apex, which makes pollen tubes an excellent model to study a large variety of dynamic events: targeted vesicle transport and docking, cell wall formation, guidance and maintenance of polarity, organelle movement, membrane recycling. We have previously shown that Rab homologues (especially those involved in the delivery of secretory vesicles to the plasma membrane) are the most abundant small GTPases in tobacco pollen tubes (Zarsky and Cvrckova, 1997) and cloned a tobacco pollen Rop (Rho of plants) homologue (Cvrckova and Zarsky, 1999). We have also shown the importance of GTPases (possibly Rop) in polarized pollen tube growth by microinjection of GTP/GDP analogues into growing pollen tubes (Elias et al., 2001). Both Rab and Rho GTPases are potential interactors of the exocyst (Sec6/8) complex, a multisubunit protein assembly involved in exocytosis that has been characterized in Saccharomyces cerevisiae and mammalian cells (reviewed in Hsu et al., 1999). The exocyst is composed of eight distinct subunits, referred to in yeast as Sec3p, Sec5p, Sec6p, Sec8p, Sec10p, Sec15p, Exo70p and Exo84p. The mammalian exocyst contains homologues of all these proteins. The complex is critical for specification of the site of vesicle docking and fusion and probably acts as a tethering complex before the steps mediated by SNARE and associated proteins. The yeast exocyst is known to interact directly with both Rab (Sec4) and Rho GTPases. Importantly, the Rho family GTPases Rho1 and Cdc42 are involved in the regulation of intracellular localization of the exocyst (Guo et al., 2001). Since the exocyst seems to be well conserved among distinct lineages of eukaryotes, it is relevant to ask whether plants also contain a similar module involved in exocytosis and secretion. Attempts to clone plant genes homologous to SEC15 via complementation of a yeast mutant with an Arabidopsis cDNA library were unsuccessful (Matsuda and Nakano, 1998). However, our searches through public sequence databases did identify genes significantly similar to all exocyst subunits (Cvrckova et al., 2001, and our unpublished data). The genome of Arabidopsis thaliana seems to code for two highly similar genes homologous to SEC3, SEC5, one homologue of SEC6, SEC8 and SEC10 each, two paralogues related to SEC15 and three copies of a putative EXO84. Surprisingly, there are as many as 23 potential Arabidopsis genes related to EXO70, although Saccharomyces, Drosophila, Caenorhabditis and perhaps also mammals possess only one copy of the EXO70 gene. This suggests that the plant exocyst might have some special features compared to other eukaryotes (Fig. 1). We have started molecular characterization of the putative Arabidopsis exocyst subunits. First we isolated a cDNA clone of the AtSec15b gene using a PCR-based library screen, and obtained cDNA clones potentially containing complete coding sequence of other exocyst * Corresponding author. Tel.: +420-2-21953179; fax: +420-2-21953306. E-mail address: zarsky@ueb.cas.cz (V. Zarsky). Cell Biology International 27 (2003) 199–201 Cell Biology International