The Arabidopsis PILZ group genes encode tubulin-folding cofactor orthologs required for cell division but not cell growth

Abstract

Plant microtubules are organized into specific cell cycle-dependent arrays that have been implicated in diverse cellular processes, including cell division and organized cell expansion. Mutations in four Arabidopsis genes collectively called the PILZ group result in lethal embryos that consist of one or a few grossly enlarged cells. The mutant embryos lack microtubules but not actin filaments. Whereas the cytokinesis-specific syntaxin KNOLLE is not localized properly, trafficking of the putative auxin efflux carrier PIN1 to the plasma membrane is normal. The four PILZ group genes were isolated by map-based cloning and are shown to encode orthologs of mammalian tubulin-folding cofactors (TFCs) C, D, and E, and associated small G-protein Arl2 that mediate the formation of ␣/␤-tubulin heterodimers in vitro. The TFC C ortholog, PORCINO, was detected in cytosolic protein complexes and did not colocalize with microtubules. Another gene with a related, although weaker, embryo-lethal phenotype, KIESEL, was shown to encode a TFC A ortholog. Our genetic ablation of microtubules shows their requirement in cell division and vesicle trafficking during cytokinesis, whereas cell growth is mediated by microtubule-independent vesicle trafficking to the plasma membrane during interphase. The microtubule cytoskeleton plays important roles in both nondividing and dividing cells of eukaryotes, assisting in vesicle trafficking and mediating proper segregation of daughter chromosomes to opposite poles during mitosis (for review, see Cole and Lippincott-Schwartz 1995;Straight and Field 2000). In fission yeast, microtubules have also been implicated in maintaining cell shape and polarity (Sawin and Nurse 1998). Higher-plant cells form their own specific microtubule arrays, such as cortical hoops of interphase microtubules that organize cell elongation (Whittington et al. 2001), and two arrays related to the plant-specific mode of cell division (for review, see Lloyd and Hussey 2001). The preprophase band forms transiently at the onset of mitosis and marks the cortical division site (for review, see Smith 2001). The phragmoplast forms at the center of the division plane in late anaphase and guides the delivery of Golgiderived vesicles. Their fusion results in the formation and lateral expansion of the cell plate that matures into a cell wall and flanking plasma membranes (for review, see Staehelin and Hepler 1996). Both preprophase band and phragmoplast also contain actin filaments; however, their specific role in plant cell division is not understood at present (Smith 2001).Microtubules are polymerized from ␣/␤-tubulin heterodimers (for review, see Nogales 2000). Newly synthesized ␣-and ␤-tubulin polypeptides undergo a sequence of folding steps catalyzed by chaperones. Initially, the tubulins are associated with the hexameric prefoldin complex that passes them on to the cytosolic chaperonin complex (Geissler et al. 1998;Vainberg et al. 1998;Hansen et al. 1999; for review, see Leroux and Hartl 2000;Llorca et al. 2000). Whereas fully func...

The rise and fall of the phragmoplast microtubule array

“…Particularly, mutational analyses have led to the identification of proteins important for cytokinesis, many of which are conserved among eukaryotes [10,11]. Inactivation of a number of factors that regulate fundamental aspects of MT dynamics certainly leads to the eventual failure in cytokinesis [12]. However, there are factors that act specifically on phragmoplast MTs and regulate the rise and fall of the phragmoplast MT array [13].…”

Silencing of Tubulin Binding Cofactor C Modifies Microtubule Dynamics and Cell Cycle Distribution and Enhances Sensitivity to Gemcitabine in Breast Cancer Cells

“…TBCC is an essential and key player required for the formation of the tubulin heterodimers by promoting the assembly of the a-and b-tubulin peptides. A study on POR gene in Arabidopsis thaliana showed that it encodes an ortholog of TBCC and plays an important role in releasing competent polymerizable heterodimers (17,18). Conversely, there are no data in the literature about the relationship between TBCC and the cell cycle or response to anti-microtubule agents in breast cancer cells.…”

Update: Plant Cortical Microtubule Arrays

“…This tubulin garage folds the a-and b-tubulin polypeptides and facilitates a/b-tubulin heterodimer formation in the presence of GTP (Steinborn et al, 2002). Both a-and b-tubulin have GTP-binding sites where the a-tubulin site traps the nucleotide within the heterodimer (Spiegelman et al, 1977).…”