Purification and characterization of 55‐kDa protein with 3,5,3′‐triiodo‐l‐thyronine‐binding activity and protein disulfide‐isomerase activity from beef liver membrane

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Abstract
Beef liver membranes were shown to have different kinds of 3,5,3′-triiodo-l-thyronine binding proteins including the 55-kDa protein which had been reported to have this activity in many cells by affinity labelling with N-bromoacetyl-3,5,3′-[125I]triiodo-l-thyronine. In order to characterize the molecular features of these binding proteins, the 55-kDa protein was purified from a beef liver membrane fraction abundant in the plasma membrane. The protein was solubilized with 0.5% Chaps and purified by chromatography on gel filtration, hydroxyapatite. and Mono Q anion-exchange columns. The purity was confirmed with reversed-phase HPLC and SDS/PAGE. Consequently, 0.4% of the total proteins in the membrane fraction was recovered as the 55-kDa protein. One fourth of the amino acid composition of this protein was Glx (14.6%) plus Asx (11.7%) and the pI of this protein was 4.5. The purified protein has triiodothyronine-binding activity with a Kd of 57 nM which is similar to the high-affinity binding site of the membranes. The anti-(55-kDa protein) sera specifically recognized the 55-kDa protein of beef, rat and human cells. The immunoglobulin G fraction of the anti-(55-kDa protein) sera inhibited triiodothyronine binding to the beef liver membrane fraction. The purified protein also showed the activity of protein disulfide-isomerase (EC 5.3.4.1) as determined by reactivating scrambled ribonuclease. These data strongly suggested that the multi-functional 55-kDa protein which has triiodothyronine-binding activity and the activity of protein disulfide-isomerase, which is also reported to be the β subunit of prolyl-4-hydroxylase, glycosylation-site-binding protein of oligosaccharyl transferase and iodothyronine 5′-monodeionidase, could be significant in the action of triiodothyronine towards the target cells.

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