The Neuron Specific Formin Delphilin Nucleates Actin Filaments but Does Not Enhance Elongation

The formin Delphilin binds the glutamate receptor, GluR{delta}2, in dendritic spines of Purkinje cells. Both proteins play a role in learning. To understand how Delphilin functions in neurons, we studied the actin assembly properties of this formin. Formins have a conserved formin homology 2 domain, which nucleates and remains associated with the fast growing end of actin filaments, influencing filament growth with input from the adjacent formin homology 1 domain. The strength of nucleation and elongation varies widely across formins. Additionally, most formins have conserved domains that regulate actin assembly through an intramolecular interaction. Delphilin is distinct from other formins in several ways: its expression is limited to Purkinje cells; it lacks autoinhibitory domains; its formin homology 1 domains has minimal proline-rich sequence. We found that Delphilin is an actin nucleator that does not accelerate elongation, although it binds tightly to the barbed end of filaments. In addition, Delphilin exhibits a preference for actin isoforms, which has not been described or systematically studied in other formins. Finally, Delphilin is the first formin studied that is not regulated by intramolecular interactions. We speculate how the activity we observe is consistent with its localization in the small dendritic spines.\n\nAbbreviations: FH1, formin homology 1; FH2, formin homology 2; DID, diaphanous inhibitory domain; DAD, diaphanous autoinhibitory domain; LTD, long term depression; PSD, post synaptic density; PDZ, post synaptic density protein (PSD95), Drosophila disc large tumor suppressor (Dlg1), and zonula occludens-1 protein (zo-1).