5-Hydroxytryptamine 4(a) receptor expressed in Sf9 cells is palmitoylated in an agonist-dependent manner

The mouse 5-hydroxytryptamine 4(a) receptor [5-HT_4(a)] was expressed with a baculovirus system in insect cells and analysed for acylation. [³H]Palmitic acid was effectively incorporated into 5-HT_4(a) and label was sensitive to the treatment with reducing agents indicating a thioester-type bond. Analysis of protein-bound fatty acids revealed that 5-HT_4(a) contains predominantly palmitic acid. Treatment of infected Sf9 (Spodoptera frugiperda) cells with BIMU8 {(endo-N-8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dehydro-2-oxo-3-(prop-2-yl)-1H-benzimid-azole-1-carboxamide}, a 5-HT₄ receptor-selective agonist, generated a dose-dependent increase in [³H]palmitate incorporation into 5-HT_4(a) with an EC₅₀ of approx. 10nM. The change in receptor labelling after stimulation with agonist was receptor-specific and did not result from general metabolic effects. We also used both pulse labelling and pulse–chase labelling to address the dynamics of 5-HT_4(a) palmitoylation. Incorporation studies revealed that the rate of palmitate incorporation was increased approx. 3-fold after stimulation with agonist. Results of pulse–chase experiments show that activation with BIMU8 promoted the release of radiolabel from 5-HT_4(a), thereby reducing the levels of receptor-bound palmitate to approximately one-half. Taken together, our results demonstrate that palmitoylation of 5-HT_4(a) is a reversible process and that stimulation of 5-HT_4(a) with agonist increases the turnover rate for receptor-bound palmitate. This provides evidence for a regulated cycling of receptor-bound palmitate and suggests a functional role for palmitoylation/depalmitoylation in 5-hydroxytryptamine-mediated signalling.