One-trial conditioning in Hermissenda results in enhanced intrinsic cellular excitability of sensory neurons in the conditioned stimulus pathway, and the phosphorylation of several proteins. Previous results demonstrated that the development of enhanced intrinsic excitability was dependent on the expression of conditioned stimulus pathway phosphoprotein-24 (Csp24), an intracellular protein containing four repeated beta-thymosin homology domains. Consistent with this, antisense oligonucleotide-mediated inhibition of Csp24 expression prevents the reduction in amplitude of the A-type transient K+ current (I(A)) and the depolarized shift in the steady-state activation curve normally produced by one-trial in vitro conditioning of isolated photoreceptors. One-trial conditioning also regulates Csp24 phosphorylation. We now show that purified recombinant Csp24 sequesters G-actin in vitro with an approximate K(d) value of 2.8 microM. We also observed a significant increase in the coprecipitation of actin with Csp24 after one-trial in vitro conditioning using antibodies directed toward either Csp24 or phospho-Csp24. Preincubation with protein kinase C (PKC) selective inhibitors attenuated the increase in Csp24 phosphorylation and coprecipitated actin observed after one-trial conditioning. Our findings indicate that the PKC signaling pathway contributes to the phosphorylation of Csp24 after one-trial conditioning, and that PKC activity modulates an association between Csp24 and actin. These data suggest Csp24 may influence intrinsic excitability by regulating cytoskeletal dynamics.