Receptor crosstalk protein, calcyon, regulates affinity state of dopamine D1 receptors

The recently cloned protein, calcyon, potentiates crosstalk between G_s-coupled dopamine D1 receptors and heterologous G_q/11-coupled receptors allowing dopamine D1 receptors to stimulate intracellular Ca²⁺ release, in addition to cAMP production. This crosstalk also requires the participating G_q/11-coupled receptors to be primed by their agonists. We examined the ability of calcyon and priming to regulate the affinity of dopamine D1 receptors for its ligands. Receptor binding assays were performed on HEK293 cell membrane preparations expressing dopamine D1 receptors either alone or in combination with calcyon. Co-expression of dopamine D1 receptor and calcyon affected neither the affinity of this receptor for antagonists nor the affinity of agonist binding to this receptor high and low-affinity states. However, the presence of calcyon dramatically decreased the proportion of the high-affinity dopamine D1 receptor agonist binding sites. This decrease was reversed by carbachol, which primes the receptor crosstalk by stimulating endogenous G_q/11-coupled muscarinic receptors. Our findings suggest that calcyon regulates the ability of dopamine D1 receptors to achieve the high-affinity state for agonists, in a manner that depends on priming of receptor crosstalk.