An alteration of an amino acid sequence in the processing site of the
insulin proreceptor by a point mutation of the
insulin receptor gene produced extreme
insulin resistance. We characterized functional properties of the unprocessed
insulin receptor in transformed lymphocytes from a patient.
Insulin binding to intact cells and to a partially purified
insulin receptor preparation was radically decreased to 20% and 18% of the control values, respectively. In competitive
insulin binding to intact cells, [LeuA3]-, [LeuB24]-, [SerB24]-
insulin, and
mini-proinsulin ([ B(1-29)-
Ala-Ala-Lys-A(1-21)]-
insulin) had the same relative binding activity in both the patient's and the control cells, but
proinsulin and
IGF-I were markedly less able to displace 125I-insulin in the patient's cells. In contrast to the study in intact cells,
proinsulin and
IGF-I as well as other
insulin analogues had the same relative binding activity to bind to the partially
lectin-purified
insulin receptor preparations from both the patient's and the control cells. As regards the signal transduction after receptor binding,
insulin-stimulated autophosphorylation of the unprocessed
insulin proreceptor occurred proportionally to the amount of decreased
insulin binding. With 0.025%
trypsin treatment, the abnormal binding characteristics and autophosphorylation were normalized through conversion to functionally normal receptors. In spite of the abnormal processing, self-association of receptors into oligomeric structures was observed in the proreceptor. These results suggest that the unprocessed
insulin proreceptor in the plasma membranes has an altered conformation which affects its binding characteristics but not its intramolecular signal transmission.