Synaptic localization of p39, a neuronal activator of cdk5.

The expression and kinase activity of cyclin dependent kinase 5 (cdk5) parallels the extent of neuronal differentiation. Cdk5 activity has been shown to be required for neurite outgrowth, cortical lamination and the overall development of the nervous system. p35 was identified as the first regulatory activator of cdk5 whose presence is required for cdk5 activation. p39 is a homolog of p35, and the only one identified in mammals thus far. We show here that p39 expression is mainly postnatal. In addition, we provide evidence for the presence of p39 at synaptic junctions through co-fractionation experiment, electron microscopy and immunostaining. The temporal and spatial expression of p39 indicate a possible role of the p39/cdk5 kinase at the synapse.