DCAMKL1 Encodes a Protein Kinase with Homology to Doublecortin that Regulates Microtubule Polymerization

Doublecortin (DCX) is a microtubule-associated protein required for neuronal migration to the cerebral cortex. DCAMKL1 consists of an N terminus that is 65 % similar to DCX throughout the entire length of DCX, but also contains an additional 360 amino acid C-terminal domain encoding a putative Ca21/calmodulin-dependent protein kinase. The homology to DCX suggested that DCAMKL1 may regulate microtubules, as well as mediate a phosphorylation-dependent signal transduction pathway. Here we show that DCAMKL1 is expressed throughout the CNS and PNS in migrating neuronal populations and overlaps in its ex-pression with DCX and microtubules. Purified DCAMKL1 asso-ciates with microtubules and stimulates polymerization of puri-fied tubulin and the formation of aster-like microtubule structures. Overexpressed DCAMKL1 leads to striking microtu-bule bundling in cell lines and cultured primary neural cells. Time-lapse imaging of cells transfected with a DCAMKL1-green fluorescent protein fusion protein shows that the microtubules associated with the protein remain dynamic. DCAMKL1 also encodes a functional kinase capable of phosphorylating myelin basic protein and itself. However, elimination of the kinase activ-ity of DCAMKL1 has no detectable effect on its microtubule polymerization activity. Because DCAMKL1 is coexpressed with DCX, the two proteins form a potentially mutually regulatory network linking calcium signaling and microtubule dynamics. Key words: neuronal migration; microtubule; kinase; microtubule-associated protein; doublecortin; lissencephaly Dynamic rearrangements of microtubules are required for many aspects of neuronal development and function, including migra-tion, process outgrowth, and synaptic plasticity (Matus, 1994; Brandt, 1996; Baas, 1999). The role of microtubules in neuronal process outgrowth has been best studied in terms of specialized neuronal microtubule-associated proteins (MAPs) such as tau an