In plants, as in animal cells, microtubules have important roles in interphase and mitotic cells. In plants during interphase, cortical microtubules appear to be connected to the plasma membrane through protein bridges. Gardiner et al. identified in Arabidopsis a tubulin-binding protein, simply called p90 for 90-kD protein, as phospholipase D-δ (PLDδ) in a screen of an Arabidopsis expression library with monoclonal antibodies originally raised against p90 isolated from tobacco. Purification of p90 by either tubulin affinity chromatography or the monoclonal antibody demonstrated enrichment in PLD activity, confirming the enzymatic activity of p90. The interaction of p90 with microtubules in interphase cells was confirmed by cosedimentation and immunolocalization experiments in tobacco cells. In cells treated with taxol to stabilize microtubules, p90 aligned with the reorganized microtubules. Arabidopsis PLDδ has a C2 calcium-binding domain, which may allow external stimuli that trigger calcium transients to regulate the association of p90 with the plasma membrane. Solubilization experiments in tobacco cells showed that p90 remained associated with the membrane following microtubule depolymerization in the presence of calcium and could be extracted from the membrane with detergent. Finally, the colocalization of p90 with microtubules persisted during mitosis with accumulation at the preprophase band and the mitotic spindle. As a microtubule binding protein and a lipid binding protein with PLD activity, p90 is poised to transmit signals from the plasma membrane to the cytoskeleton.
J. C. Gardiner, J. D. I. Harper, N. D. Weerakoon, D. A. Collings, S. Ritchie, S. Gilroy, R. J. Cyr, J. Marc, A 90-kD phospholipase D from tobacco binds to microtubules and the plasma membrane. Plant Cell 13, 2143-2158 (2001). [Abstract] [Full Text]