||Cdk5 in conjunction with its neuronal activator p35 plays a pivotal role in neuronal differentiation and neuronal migration during the brain development. In this thesis work we found that p35, a neuron-specific activator of Cdk5, associates with Ribosomal S6 kinase 1 (S6K1). In addition, Cdk5 phosphorylates S6K1 at Ser411 in vitro and in vivo. S6K1, a key regulator of mRNA translation, plays an important role in cell cycle progression through the G1 phase of proliferating cells and in the synaptic plasticity of terminally differentiated neurons. Activation of S6K1 involves the phosphorylation of its multiple Ser/Thr residues, including the proline-directed sites (Ser-411, Ser-418, Thr-421, and Ser-424) in the autoinhibitory domain near the C terminus. Phosphorylation at Thr-389 is a crucial event in S6K1 activation. Here, we report that S6K1 phosphorylation at Ser-411 is required for the rapamycin-sensitive phosphorylation of Thr-389 and the subsequent activation of S6K1. Mutation of Ser-411 to Ala ablated insulin-induced Thr-389 phosphorylation and S6K1 activation, whereas mutation mimicking Ser-411 phosphorylation did not show any effect. Furthermore, phosphomimetic mutation of Thr-389 overcame the inhibitory effect of the mutation S411A. Thus, Ser-411 phosphorylation regulates S6K1 activation via the control of Thr-389 phosphorylation. In nervous system neurons, Cdk5-p35 kinase associates with S6K1 via the direct interaction between p35 and S6K1 and catalyzes S6K1 phosphorylation specifically at Ser-411. Inhibition of the Cdk5 activity or suppression of Cdk5 expression blocked S6K1 phosphorylation at Ser-411 and Thr-389, resulting in S6K1 inactivation. Similar results were obtained by treating asynchronous populations of proliferating cells with the CDK inhibitor compound roscovitine. Altogether, our findings suggest a novel mechanism by which the CDK-mediated phosphorylation regulates the activation of S6K1. Microtubules are one of the main cytoskeletal elements in eukaryotic cells, intimately regulating cell shape and mobility. The assembly of tubulin subunits into microtubules is promoted by microtubule-associated proteins (MAPs). Here we show that p35 binds directly to microtubules. Microtubule polymers but not the α/β-tubulin heterodimer block p35 interaction with Cdk5 and therefore inhibit Cdk5-p35 activity. p25, a neurotoxin-induced truncated form of p35, does not have the tubulin and microtubule-binding activities, and thus Cdk5-p25 is inert to the inhibitory effect of microtubules. In cultured cortical neurons, a significant proportion of p35 localizes to microtubules. However, Cdk5 phosphorylation of p35 leads to dissociate from microtubules in an intermolecular kinase mechanism. Together, these findings suggest that p35 is a microtubule-associated protein that modulates microtubule dynamics. Also, microtubules play an important role in the control of Cdk5 activation.