Structural analogs of human insulin-like growth factor (IGF) I with altered affinity for type 2 IGF receptors.
We have used site-directed mutagenesis of a synthetic gene for insulin-like growth factor (IGF) I to prepare three analogs in which specific residues in the A region are replaced with the corresponding residues in the A chain of insulin. The analogs are [Ile41, Glu45, Gln46, Thr49, Ser50, Ile51, Ser53, Tyr55, Gln56]IGF I (A chain mutant), in which residue 41 is changed from threonine to isoleucine and residues 42 to 56 of the A region are replaced, [Thr49, Ser50, Ile51]IGF I, and [Tyr55, Gln56]IGF I. These analogs are all equipotent to IGF I at the type 1 IGF receptor in human placental membranes, and in stimulating the incorporation of [3H]thymidine into DNA in the rat vascular smooth muscle cell line A10. However, the A chain mutant and [Thr49, Ser50, Ile51]IGF I have greater than 20-fold lower relative affinity for the type 2 IGF receptor of rat liver membranes, respectively. In contrast, [Tyr55, Gln56]IGF I has 7-fold higher affinity than IGF I for the type 2 IGF receptor. Residues 49, 50, and 51 in IGF I are Phe-Arg-Ser and are strictly conserved in IGF II. Residues 55 and 56 of IGF I and the corresponding residues in IGF II are Arg-Arg and Ala-Leu, respectively. Thus, the presence of the charged residues at these positions in IGF I appears to be responsible, in part, for the lower affinity of IGF I for the type 2 IGF receptor. In addition to the alterations in affinity for the type 2 IGF receptor, the A chain mutant has a 7-fold increase in affinity for insulin receptors, and [Thr49, Ser50, Ile51]IGF I has a 4-fold lower affinity for acid-stable human serum binding protein. These data strongly suggest that specific determinants in the A region of IGF I are important for maintaining binding to the type 2 IGF receptor, and that these determinants are different from those required for maintaining high affinity for the type 1 IGF receptor.
Cascieri
M A
MA
Department of Biochemical Endocrinology, Merck Sharp & Dohme Research Laboratories, Rahway, New Jersey 07065.
Chicchi
G G
GG
Applebaum
J
J
Green
B G
BG
Hayes
N S
NS
Bayne
M L
ML
D000595:Amino Acid Sequence; D001483:Base Sequence; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D005796:Genes; D005813:Genes, Synthetic; D005822:Genetic Vectors; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000031:analogs & derivatives* / Q000235:genetics* / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D010920:Placenta / Q000378:metabolism; D011247:Pregnancy; D011487:Protein Conformation; D011972:Receptor, Insulin / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017451:Receptors, Somatomedin; D013002:Somatomedins / Q000031:analogs & derivatives* / Q000235:genetics* / Q000378:metabolism*
D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D011972:Receptor, Insulin