Dual effects of insect nAChR chaperone RIC-3 on hybrid receptor: Promoting assembly on endoplasmic reticulum but suppressing transport to plasma membrane on Xenopus oocytes

Resistance to inhibitors of cholinesterase (RIC) −3 promotes the maturation (folding and assembly) of neuronal nicotinic acetylcholine receptors (nAChRs) as a molecular chaperone. The modulation effects of RIC-3 on homomeric α7 nAChRs are always positive, but its effects on heteromeric subtypes are inconsistent among reports. In this study, five RIC-3 isoforms were identified from Locusta migratoria. Four isoforms showed obvious effects on hybrid receptor Locα1/rβ2 expressed in Xenopus oocytes. As a representative, the co-expression of RIC-3v4 exhibited the decreased agonist responses (Imax) on oocytes, lower specific [3H]epibatidine binding (Bmax) on plasma membrane protein (PMP), and reduced subunit levels in PMP, which showed that the mature Locα1/rβ2 on the plasma membrane was decreased by the co-expression of RIC-3. In contrast, the [3H]epibatidine binding and mature Locα1/rβ2 levels in the endoplasmic reticulum membrane protein (ERMP) were much increased when co-expressing with RIC-3v4. The [3H]epibatidine binding and mature Locα1/rβ2 levels in total membrane protein (TMP) gave the similar results as that in ERMP. Taking data together, the results showed that the co-expression of RIC-3 increased the mature Locα1/rβ2 receptor levels on ER of Xenopus oocytes, but these mature receptors were mostly kept on ER and suppressed to transport to plasma membrane.