Abundant microtubules (MTs) were present both in protoplasts isolated from tobacco BY-2 cells and on membrane ghosts prepared from such protoplasts. However, only a few MTs or none at all were observed on membrane ghosts prepared from protoplasts pretreated with protease, trypsin or chymotrypsin, although abundant MTs were present in protease-pretreated protoplasts. These observations suggest that the digestion of the extracellular portion of transmembrane protein(s) results in the dissociation of cortical MTs from the plasma membrane. Exogenously applied extensin or poly-L-lysine increased the cold-stability of cortical MTs in the control protoplasts, but not in protease-pretreated protoplasts. It appears that the transmembrane protein(s) is also involved in the stabilization of cortical MTs by extensin or poly-L-lysine. Cortical MTs in BY-2 cells were arranged parallel to one another and were resistant to cold. Treatment with protease rendered the MTs sensitive to cold and often disturbed the parallel array of cortical MTs. These results suggest that transmembrane protein(s) is involved in the arrangement and stabilization of cortical MTs in tobacco BY-2 cells.

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