A cytoplasmic thyroid hormone binding protein: characterization using monoclonal antibodies

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Previously purified cellular thyroid hormone binding protein (p58) from human carcinoma cells.
• Further characterization of p58's binding, molecular properties, and subcellular localization is needed.

Purpose of the Study:

• To characterize the binding characteristics of purified p58 to thyroid hormones.
• To investigate the molecular properties, including post-translational modifications and degradation rate.
• To determine the subcellular localization of p58 using immunocytochemical and fractionation studies.

Main Methods:

• Thyroid hormone binding assays with purified p58.
• Generation and characterization of monoclonal antibodies (J11, J12) against p58.
• Analysis of post-translational modifications (glycosylation, sulfation, phosphorylation).
• Determination of cellular degradation rate (t1/2).
• Immunocytochemistry and subcellular fractionation.

Main Results:

• p58 binds 3,3',5-triiodo-L-thyronine (T3) with a Kd of 24.3 nM and shows similar binding to L-thyroxine.
• Affinity for D-T3 and reverse-T3 is 4- and 20-fold less than T3, respectively.
• Monoclonal antibodies J11 and J12 exhibit distinct species specificities.
• p58 is not glycosylated, sulfated, or phosphorylated, with a degradation rate t1/2 of approximately 2.1 hours.
• p58 is localized in the nonmembranous cytoplasm (cytosol), with >95% reactivity in the 110,000g supernatant.

Conclusions:

• p58 exhibits specific binding characteristics for thyroid hormones.
• Monoclonal antibodies provide tools for studying p58 across various species.
• p58 lacks common post-translational modifications and has a relatively short degradation half-life.
• The cytoplasmic localization of p58 is confirmed, suggesting a role in intracellular thyroid hormone transport or action.