| Abstract
| - The δ opioid binding sites in subcellular fractionsfrom NG108-15 cells were characterizedwith respect to their relative molecular size and levels underconditions of receptor adaptation.125I-β-Endorphin was cross-linked to preparations enriched in plasma membranes(P20), nuclear membranes ornuclear matrices. Five cross-linked bands appear in allsubcellular fractions. The largest molecular sizereaction product in nuclear matrix preparations (∼72 kDa) differedfrom that in the other two fractions(∼83 kDa). Immunoblot analyses with an antibody to the δ opioidreceptor gave a P20 band patternsimilar to that for the corresponding cross-linked products. Todetermine which cross-linked products inP20 are glycoproteins, labeled membranes were solubilizedand purified by wheat germ agglutininchromatography. The absence of a ∼36 kDa band after purificationsuggests that this product is not aglycoprotein. The remaining four bands were present inN-acetyl-d-glucosamine eluates, althoughtheir% distribution changes in favor of the largest molecular size band(∼83 kDa). Immunoblotting of theeluate gave a single diffuse band at ∼73 kDa, suggesting the nativeglycoprotein has a molecular size inthe 70−80 kDa range. Etorphine-induced desensitization of cellsurface receptors increased the amountof some cross-linked products associated with nuclear membranes.The same treatment did not affect therelative density of the four larger molecular size bands inP20, but increased the density of the ∼26kDaproduct two fold. Etorphine-induced down-regulation evoked anelevation of cross-linked products innuclear matrix preparations, while all band densities ofP20 were diminished. These results suggestthatnuclear matrix associated opioid binding sites represent internalized,truncated forms of the glycosylatedδ opioid receptor found in P20.
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