Molecular characterization and localization of the RIC-3 protein, an effector of nicotinic acetylcholine receptor expression.

The RIC-3 protein acts as a regulator of acetylcholine nicotinic receptor (nAChR) expression. In Xenopus laevis oocytes the human RIC-3 (hRIC-3) protein enhances expression of alpha7 receptors and abolishes expression of alpha4beta2 receptors. In vitro translation of hRIC-3 evidenced its membrane insertion but not the role as signal peptide of its first transmembrane domain (TMD). When the TMDs of hRIC-3 were substituted, its effects on nAChR expression were attenuated. A certain linker length between the TMDs was also needed for alpha7 expression enhancement but not for alpha4beta2 inhibition. A combination of increased alpha7 receptor steady state levels, facilitated transport and reduced receptor internalization appears to be responsible for the increase in alpha7 membrane expression induced by hRIC-3. Antibodies against hRIC-3 showed its expression in SH-SY5Y and PC12 cells and its induction upon differentiation. Immunohistochemistry demonstrated the presence of RIC-3 in rat brain localized, in general, in places where alpha7 nAChRs were found.