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Dyrk1A is a regulator of dendrite morphology that phosphorylates doublecortin, a microtubule‐bunding protein

To understand the phosphorylation events underlying axon/dendrite growth, we screened a cDNA library of kinases and phosphatases by overexpression in primary hippocampal neurons. One of the hits was the dual specificity tyrosine (Y) phosphorylation‐regulated kinase, Dyrk4. Overexpression of Dyrk4 increased both the number of primary neurites and the number of dendritic branches. A close relative of Dyrk4, Dyrk1A, increased dendritic branching and greatly affected the morphology of dendritic growth cones. Moreover, overexpression of several members of the Dyrk family (Dyrk1A, Dyrk2 and Dyrk3) perturbed distribution of doublecortin (DCX), a microtubule‐binding protein enriched in axonal and dendritic growth cones. Since DCX function is regulated by phosphorylation we therefore investigated the possibility that it can be a target of DYRK kinases. Co‐expression of DCX with Dyrk1A, Dyrk2 or Dyrk3, but not Dyrk4, in HEK293 cells produced lower mobility bands recognized by anti‐phospho‐DCX antibody. Sequence analysis of the DCX coding region revealed a putative phosphorylation site for DYRK kinases at serine 306 (S306). It's mutation to alanine eliminated shifted phospho bands, suggesting that S306 is indeed a target of these kinases. A more comprehensive mutational analysis suggests that phosphorylation of S306 by Dyrk kinases may prime phosphorylation by Cdk5.