Neuronal protein 22 colocalises with both the microtubule and microfilament cytoskeleton in neurite-like processes.

The expression of human neuronal protein 22 (hNP22) is up-regulated in the superior frontal cortex of chronic alcoholics. hNP22 shares significant homology with a number of proteins implicated in bundling of actin filaments. In addition, it contains domains similar to those found in microtubule-associated proteins. We investigated the ability of hNP22 to induce cytoskeletal changes by overexpression in Chinese hamster ovary cells. Overexpression of hNP22 resulted in process formation in these cells that increased upon treatment with cytochalasin D, an actin depolymerising agent. Transfection of mutant hNP22 containing either a deletion of the putative actin-binding domain or deletion of a consensus protein kinase C (PKC) phosphorylation site (Ser-180) failed to induce process formation. In contrast, a mutation to mimic persistent PKC phosphorylation resulted in a cellular morphology similar to that seen in wild-type hNP22 transfections. This observation suggests that hNP22 requires phosphorylation at Ser-180 by PKC to induce cytoskeletal rearrangements. hNP22 was also observed to colocalise with actin and tubulin in processes of transfected cells. An hNP22-specific antibody specifically immunoprecipitated a complex including tubulin from human brain indicating that hNP22 binds directly to microtubules. Taken together, this data suggests that NP22 is part of a signaling complex that associates with cytoskeletal elements to regulate neuronal morphology.