Nonpermeant covalent labels in analytical studies of lymphocyte membrane proteins

Determination of the sidedness of the carboxy-terminus of the Na+/K(+)ATPase alpha-subunit using lactoperoxidase iodination

The orientation of the carboxy-terminal pair of tyrosines of the Na+/K(+)-ATPase alpha-subunit with respect to the plane of the plasma membrane was determined. The approach was based on lactoperoxidase-catalysed radioiodination of the tyrosine residues accessible on the surface of the enzyme molecule in intact cells of a pig kidney embryonic cell line and those accessible in a broken plasma membrane fraction and in isolated membrane-bound Na+/K(+)-ATPase. The labeled alpha-subunit was isolated by SDS gel electrophoresis followed by electroblotting. Then the COOH-terminal amino acids were hydrolyzed by carboxypeptidases B and Y. Radioactivity and quantitative analysis of the protein and released amino acids showed that the COOH-terminal tyrosine residues of the alpha-subunit were only accessible to modification only when lactoperoxidase had access to the inner side of the plasma membrane. Therefore, the COOH-terminus of the Na+/K(+)-ATPase alpha-subunit is located on the cytoplasmic surface of the pump molecule and its polypeptide chain must have an even number of transmembrane segments.

A simple method for the radioactive iodination of CD4 molecules

A method for the vectorial radioiodination of CD4 and other membrane proteins having few or no tyrosine residues in the extracytoplasmic domains is described. Incubation of the cells with sulfosuccinimidyl (hydroxyphenyl) propionate (sulfo-SHPP), a water-soluble derivative of the Bolton-Hunter reagent results in the coupling of hydroxyphenyl groups to free amino groups of cell surface proteins and these groups are then vectorially radioiodinated using 1,3,4,6-tetrachloro-3 alpha,6 alpha-diphenylglycoluril (Iodogen). The method is highly efficient, gentle, fast, simple, and does not require the previous radiolabelling of the Bolton-Hunter reagent.