FIGURE 4.

p35 23R activates Cdk5 as does p35. A, binding to and activation of Cdk5 by p35 23R. p35 (WT) or p35 23R expressed in Neuro2a cells was immunoprecipitated with anti-Cdk5, and their binding of p35 23R was examined by the immunoblotting of p35 in the Cdk5 immunoprecipitates (IP). The kinase activity was measured by the phosphorylation of histone H1 (H1). Histone H1 phosphorylation is quantified in the right panel (means ± S.E., n = 3). B, phosphorylation of p35. Phosphorylation of p35 expressed in Neuro2a cells was examined in the presence or absence of Cdk5-p35 using Phos-tag SDS-PAGE (lower panel). The phosphorylation states of the respective bands are indicated at the left side of the blot, based on the results by Hosokawa et al. (38). The upper two panels are immunoblotted with anti-p35 and anti-Cdk5 after Laemmli SDS-PAGE.