Identification of cholecystokinin-B/gastrin receptor domains that confer high gastrin affinity: utilization of a novel Xenopus laevis cholecystokinin receptor.

A hallmark of the mammalian brain cholecystokinin (CCK) receptor, CCK-B/gastrin (CCK-BR), is its high affinity for two structurally related peptides, CCK and gastrin. Previous radioligand binding experiments suggested that the predominant CCK receptor from Xenopus laevis brain shares high affinity for sulfated cholecystokinin octapeptide but has > or = 1000-fold lower affinity for gastrin. To determine the molecular basis for this pharmacological divergence between mammalian and lower vertebrate receptors, we isolated a cDNA encoding the X. laevis brain CCK receptor (CCK-XLR). CCK-XLR shares approximately 50% homology at the amino acid level with both the human CCK-BR and the peripheral CCK-A receptor subtypes. The recombinant X. laevis receptor has a distinct pharmacological profile of agonist and antagonist affinities and as such offers a useful tool for structure-function studies. We used CCK-XLR to map the human CCK-BR domains that confer high affinity for gastrin. A series of chimeric CCK-BR/CCK-XLR constructs was generated and pharmacologically characterized. While maintaining wild-type affinity for sulfated cholecystokinin octapeptide, receptors with increasing amino-terminal contributions from CCK-BR demonstrated a stepwise increase in gastrin affinity. Further dissection of the amino-terminal third of the human receptor, a domain that confers a > 250-fold increase in gastrin affinity, revealed the importance of interactions among at least three subdomains. Additional structural requirements for gastrin affinity mapped to a segment spanning transmembrane domains IV and V.