E. Di Paolo et al., Mutations of aromatic residues in the first transmembrane helix impair signalling by the secretin receptor, RECEPT CHAN, 6(4), 1999, pp. 309-315
The secretin amino terminal residues are essential for high affinity bindin
g to the cognate receptor and for the subsequent activation of adenylate cy
clase. It has been already established that two basic residues of the recep
tor TM 2 are involved in the interaction with aspartate 3 of the ligand. Th
e present work investigated the hypothesis that two conserved tyrosine resi
dues of the TM 1 (Tyrosines 124 and 128) could also participate to the posi
tioning of the amino terminus of the ligand. Tyrosines 124 and 128 were mut
ated into alanine and histidine residues, and the properties of the mutant
receptors, expressed in CHO cells, were compared with those of the wild-typ
e receptor. Mutation of tyrosine 124 to Ala or His decreased the affinity o
f the receptor for secretin, [Glu(3)]secretin, [Asn(3)]secretin and the sec
retin fragment 2-27, and reduced the intrinsic activity of [Asn(3)]secretin
. Mutation of tyrosine 128 to Ala, but not to His reduced 50-fold secretin
and [Asn(3)]secretin affinity but only 3-fold that of[Glu(3)]secretin. Secr
etin and [Glu(3)]Sn were equipotent in that mutant receptor. These results
suggested that tyrosine 128 of the secretin receptor interacted directly wi
th the [Asp(3)] residue of secretin and thus that the amino terminal domain
of secretin interacts with amino acids buried in both the TM 1 and TM 2 he
lices.