b'binding the molecule GTP and by proteins associated with growing ends, creating a stabilizing cap. When severing occurs in vitro behind the stabilizing cap, the new plus ends shrink. However, that is notmicrotubuleswhat is observed in living cellsthe new plus ends typically grow. Models indicate that end growth after severing is required to create a new population ofExpansion 35microtubes. The required stabilizing factors, however, are unknown.cell wall cellulosethis reorganization is driven by creating new microtubulesPlant cells are under high internal pressure (white arrows), which pushes against a rigid cell wall. An organized microtubule by an unexpectedcytoskeleton (blue) directs cell wall synthesis (red) and loosening, determining how the wall yields to create the cells shape. processImage courtesy David Ehrhardt, Carnegie Institution for ScienceEhrhardt and Carnegie colleagues Jelmer J.GrowthLindeboom, Masayoshi Nakamura, and Ankit Walia used the model plant Arabidopsis thaliana toStabilizingunderstand the stabilizers behind the GTP-cap, whereSevering G-capsevering occurs. They looked at candidate proteins that accumulate at the plus ends and asked whether one or more of them might support the creation ofCLASPnew microtubule arrays following light stimulation.StabilizationThey selectively deactivated candidate proteins by mutation and compared the mutants to normal plants using time-lapse imaging of the microtubule arrays. Combined with analysis by computational modeling,Growththey revealed that the protein CLASP is a potent and specific stabilizer of plus ends created by severing inShrinkagevivo and that this activity is required to build a new array after light stimulation.Plant cell growth is directed by shifting arrays of hollow cylinders of protein filaments called microtubules (top). Microtubules are The researchers are currently investigating theassembled from pairs of tubulin proteins. When first assembled, interactions among neighboring tubulin pairs are strengthened by mechanisms by which CLASP stabilizes the cutthe binding of a small molecule called GTP. This stabilizing G-cap ends and promotes regrowth. These studies havepromotes continued filament growth. When plant cells in the implications for understanding cellular functionplant axis perceive light, new microtubules are created by severing existing ones (second from top), reorienting the microtubule array in animals, where CLASP is conserved, and whereand modifying cell growth. However, this severing occurs behind the important cell types, including neurons, areG-cap. The protein CLASP steps in to stabilize the new ends (third proposed to create microtubules arrays by severingfrom top) that severing creates preventing them from shrinking and promoting their growth to create the new array (bottom). and regrowth. Image courtesy David Ehrhardt, Carnegie Institution for Science'