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We have shown that gastrin and cholecystokinin octapeptide (CCK-8) are differently coupled to G protein (GTP-binding protein) through type B cholecystokinin receptors in guinea-pig brain membranes and Jurkat cells. Indeed, the gastrin-13 binding affinity is strongly reduced by stable guanyl nucleotides, whereas CCK-8 binding is only slightly affected. In order to determine the structural requirements regulating such coupling, we have synthesized several gastrin and cholecystokinin fragments (sulphated or unsulphated) elongated at the N-terminus of the common C-terminal tetrapeptide. We investigated their interaction with CCKB receptors in guinea pig brain membranes and Jurkat cells and their involvement in the G protein coupling. Their apparent binding affinities to CCKB receptors were measured by inhibition of [125I]Bolton Hunter-CCK-8 (3-[125I]iodo-4-hydroxyphenyl)propionyl-CCK-8) binding in the presence or absence of GTP gamma S (guanosine 5'-O-(3-thio)triphosphate) or aluminum tetrafluoride (AlF4-). Activation of the G proteins by GTP gamma S or AlF4- led to a decrease in binding affinity for the gastrin related peptides, the common CCK-gastrin C-terminal forms, the cholecystokinin hexapeptide and the unsulphated cholecystokinin heptapeptide. Sulphated CCK-7, CCK-8, and cionin apparent binding affinities were not affected. These finding indicated that the sulphated tyrosine in position 7 in CCK (as counted from the C-terminus), provides the cholecystokinin selectivity for the CCKB receptor compared to gastrin. The results are discussed with the aim to better clarify the physiological relevance of gastrin and cholecystokinin toward CCKB receptors and their related intracellular events.