Phosphorylation of Microtubule-associated Protein SB401 from Solanum berthaultii Regulates Its Effect on Microtubules

We reported previously that the protein SB401 from Solanum berthaultii binds to and bundles both microtubules and F-actin. In the current study, we investigated the regulation of SB401 activity by its phosphorylation. Our experimental results showed that the phosphorylation of SB401 by casein kinase II (CKII) downregulates the activities of SB401, namely the bundling of microtubules and enhancement of the polymerization of tubulin. However, phosphorylation of SB401 had no observable effect on its bundling of F-actin. Further investigation using extract of potato pollen indicated that a CKII-like kinase may exist in potato pollen. Antibodies against CKII alpha recognized specifically a major band from the pollen extract and the pollen extract was able to phosphorylate the SB401 protein in vitro. The CKII-like kinase showed a similar ability to downregulate the bundling of microtubules. Our experiments demonstrated that phosphorylation plays an important role in the regulation of SB401 activity. We propose that this phosphorylation may regulate the effects of SB401 on microtubules and the actin cytoskeleton.