title: Calmodulin binding and Cdk5 phosphorylation of p35 regulate its effect on microtubules.
abstract: In the nervous system, Cdk5 and its neuronal activator p35 are involved in the control of various activities, including neuronal differentiation and migration. Recently, we have reported that p35 is a microtubule-associated protein that regulates microtubule dynamics ( Hou, Z., Li, Q., He, L., Lim, H. Y., Fu, X., Cheung, N. S., Qi, D. X., and Qi, R. Z. (2007) J. Biol. Chem. 282, 18666-18670 ). Here we present two regulatory modes of p35 function as a microtubule-associated protein. First, p35 is Ca(2+)-dependent calmodulin (CaM)-binding protein. The CaM- and microtubule binding domains are localized to overlapping regions at the N terminus of p35. Within the CaM-binding region, Ala substitution for Trp-52 abolishes the CaM-binding activity, corroborating specific CaM-binding of p35. Furthermore, CaM blocks p35 association with microtubules in a Ca(2+)-specific manner, suggesting that p35 may be involved in the Ca(2+)/CaM-mediated inhibition of microtubule assembly. Second, p35 phosphorylation by Cdk5 interferes with the microtubule-binding and polymerizing activities of p35. Using a mutational approach, we found that only phosphorylation at Thr-138, one of the two residues primarily phosphorylated in vivo, inhibits the polymerizing activity. In PC12 cells, expression of p35 promotes nerve growth factor-induced neurite outgrowth under a Cdk5 inhibitory condition. Such p35 activity is impaired by the phosphomimetic mutation of Thr-138. These data suggest that Thr-138 phosphorylation plays a critical role in the control of the p35 functions in microtubule assembly and neurite outgrowth.
authors.affiliation: Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
authors.forename: Lisheng
authors.initials: L
authors.lastname: He
authors.forename: Zhibo
authors.initials: Z
authors.lastname: Hou
authors.forename: Robert Z
authors.initials: RZ
authors.lastname: Qi
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002147:Calmodulin / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D051360:Cyclin-Dependent Kinase 5 / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation; D006801:Humans; D008870:Microtubules / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010766:Phosphorylation; D011485:Protein Binding; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D002147:Calmodulin; D051360:Cyclin-Dependent Kinase 5; D002118:Calcium