Plant Cytokinesis – Insights Gained from Electron Tomography Studies 265
proteins (Rose et al. 2004), and three of these proteins – AtUso1 (Kang and
Staehelin, submitted), AtGRIP (Gilson et al. 2004), AtCASP (Renna et al. 2005)
- have been shown to localize to budding COPI vesicles and/or to Golgi stacks
(Golgi matrix). Yet to be determined is which type of long-coiled coil of pro-
tein produces the scaffolding structure of the CPAM.
In other systems, long coiled-coil proteins are recruited to specific mem-
brane systems through activated GTP-binding proteins and lipid species, and
this recruitment has been postulated to help each membrane compartment
create and express its own identity (Munro 2004). Once assembled, the scaf-
fold can recruit additional cytosolic proteins that facilitate the binding of
specific vesicles to this structure, and catalyze the vesicle budding and vesi-
cle fusion reactions that accompany the membrane transformation processes
characteristic of each type of membrane compartment.
4.4
The CPAM is an Affinity Matrix that Assembles and Organizes
the Molecules Required for Vesicle Tethering and Vesicle Fusion
The hypothesis that the CPAM recruits Golgi-derived vesicles, enzymes and
regulatory molecules to the cell plate assembly site is based on the finding
that specific types of molecular assemblies involved in cell plate formation
localize exclusively to this structure. For example, it has been reported that
many cell plate-forming vesicles both inside and outside the CPAM carry
∼ 35 nm long, L-shaped appendages, whereas paired vesicles held together by
Y-shaped linkers of the same length can only be detected within the CPAM
region (Fig. 6; Seguí-Simarro et al. 2004). Based on their structural and func-
tional similarities with the exocyst complexes of mammalian and yeast cells
(Terbush et al. 1996, 2001; Hsu et al. 1998), both of which are involved in po-
larized secretion, it has been postulated that that the Y-shaped complexes are
completed vesicle tethering complexes, and the L-shaped precursors that lack
the final protein(s) needed for the tethering function (Seguí-Simarro et al.
2004). Since the completed tethers are seen only inside the CPAM, it is likely
that the protein(s) needed to complete the tethers are also found only inside
this matrix. Furthermore, because Sec3 is an exocyst complex protein that
in other systems is recruited to target membranes (Finger et al. 1998), the
plant homologue of Sec3 could be the subunit that actually gives rise to the
Y-shaped tethering complexes within the CPAM. Once tethered by an exocyst
complex, the vesicles are induced to fuse through interactions between pro-
teins associated withv-andt-SNARE complexes (Waizenegger et al. 2000; see
also Chap. 13). Several SNARE proteins have also been localized to the cell
plate forming regions of phragmoplasts (Jürgens 2005a).
Putative homologues of all of the exocyst protein subunits, including Sec3,
have been identified in theArabidopsisgenome (Cvrckova et al. 2001; Jurgens
and Geldner 2002; Elias et al. 2003; Seguí-Simarro et al. 2004; Cole et al. 2005).