Mutations in the
growth hormone receptor (GHR) gene can cause
growth hormone (GH) resistance. Given the sequence homology between the extracellular domain of the GHR and a soluble
GH-binding protein (GH-BP), it is remarkable that GH-BP binding activity is absent from the serum of patients with Laron-type GH insensitivity, a hereditary form of severe
dwarfism. We have previously identified a mutation within the extracellular domain of this receptor, replacing
phenylalanine by
serine at position 96 of the mature
protein, in a patient with
Laron syndrome. We have now investigated the effect of this
Phe----Ser substitution on
hormone binding activity by expressing the total human GHR
cDNA and mutant form in eukaryotic cells. The wild-type
protein expressed was able to bind GH but no plasma membrane binding was detectable on cells transfected with the mutant
cDNA; this was also the case of cells transfected with a Phe96----Ala mutant
cDNA, suggesting that the lack of binding activity is not due to a posttranslational modification of
serine. Examination of the variant
proteins in subcellular fractions revealed the presence of specific GH binding activity in the lysosomal fraction, whereas immunofluorescence studies located
mutant proteins in the cytosol. Our findings suggest that these mutant GHRs fail to follow the correct intracellular transport pathway and underline the potential importance of this
phenylalanine residue, which is conserved among the GH,
prolactin, and
erythropoietin receptors that belong to the same
cytokine receptor superfamily.