Biosynthesis of the insulin receptor in rat adipose cells. Intracellular processing of the Mr-190 000 pro-receptor

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Abstract
We investigated the biosynthesis of the insulin receptor in primary cultures of isolated rat adipose cells. Cells were pulse-chase-labelled with [3H]mannose, and at intervals samples were homogenized. Three subcellular membrane fractions were prepared by differential centrifugation: high-density microsomal (endoplasmic-reticulum-enriched), low-density microsomal (Golgi-enriched), and plasma membranes. After detergent solubilization, the insulin receptors were immunoprecipitated with anti-receptor antibodies and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and autoradiography. After a 30 min pulse-label [3H]mannose first appeared in a band of Mr 190,000. More than 80% of the Mr-190,000 component was recovered in the microsomal fractions. Its intensity reached a maximum at 1 h in the high-density microsomal fraction and at 2 h in the low-density microsomal fraction, and thereafter declined rapidly (1/2 approx. 3 h) in both fractions. In the plasma-membrane fraction, the radioactivity in the major receptor subunits, of Mr 135,000 (.alpha.) and 95,000 (.beta.), rose steadily during the chase and reached a maximum at 6 h. The Mr-190,000 precursor could also be detected in the high-density microsomal fraction by affinity cross-linking to 125I-insulin. In the presence of monensin, a cationic ionophore that interferes with intracellular transport within the Golgi complex, the processing of the Mr-190,000 precursor into the .alpha. and .beta. subunits was completely inhibited. Our results suggest that the Mr-190,000 pro-receptor originates in the endoplasmic reticulum and is subsequently transferred to the Golgi complex. Maturation of the pro-receptor does not seem to be necessary for the expression of the insulin-binding site. Processing of the precursor into the mature receptor subunits appears to occur during the transfer of the pro-receptor from the Golgi complex to the plasma membrane.