Characterization of a N-bromoacetyl-L-thyroxine affinity-labeled 55-kilodalton protein as protein disulfide isomerase in cultured glial cells.

In glial cells, thyroid hormone regulates the polymerization state of the actin cytoskeleton by a mechanism that does not require protein synthesis or the nuclear T3 receptor. Using the affinity label N-bromoacetyl-L-T4, we identified a thyroid hormone-binding protein of 55 kDa (glial-p55) in cultured glial cells that is unique from the type II iodothyronine 5'-deiodinase and which exhibits a T4-dependent shift from a membrane-associated pool to the F-actin cytoskeleton. In a number of other cell types, a 55-kDa thyroid hormone-binding protein has been identified and shown to be a subunit of the enzyme protein disulfide isomerase (PDI). In this study we have characterized glial-p55 and compared it with purified rat hepatic PDI. Glial-p55 appears to be identical to PDI by peptide fragmentation analysis, using multiple methods. The hydrodynamic properties of glial-p55, determined by molecular sieve chromatography and sucrose density centrifugation, showed that this protein has a native molecular mass of 115 kDa, which is in close agreement with that reported for PDI. Like PDI, glial-p55 is an acidic protein with a pI of 4.8 or less. Thus, the 55-kDa thyroid hormone-binding protein in glial cells appears to be PDI.