Recently we reported the deletion of Lys-121 in one allele of the insulin receptor gene from a child with severe insulin resistance. In the present work, this mutant receptor (M121) was shown to have an abnormal sensitivity to temperature and an alteration in "negative cooperativity." In contrast to the wild-type receptor (HIRC), insulin binding by the M121 receptor was rapidly and irreversibly lost at temperatures above 30 degrees C with the phosphorylated form of the receptor being more temperature-sensitive than the nonphosphorylated form. Although insulin binding activity was lost, Western analysis and other studies showed that the mutant receptor remained intact. Measurements of 125I-insulin dissociation at 21 degrees C in the presence of native insulin (an estimate of negative cooperativity) demonstrated a difference between the mutant and wild-type receptor. Insulin dissociation from the mutant receptor was not as pronounced as that found with the wild-type receptor. Thus, an abnormality in insulin binding by the mutation was evident at lower "permissive" temperatures. The results of these and other studies argue that Lys-121 occupies an important position for the regulation of insulin receptor conformation. This regulation apparently influences negative cooperative interactions with insulin and modulates signal transduction.