DEGRADATION AND RECYCLING OF THE SUBSTRATE-BINDING SUBUNIT OF TYPE-IIIODOTHYRONINE 5'-DEIODINASE IN ASTROCYTES

Thyroxine dynamically regulates levels of type II iodothyronine 5'-dei odinase (5'D-II) by modulating enzyme inactivation and targeting the e nzyme to different pathways of internalization. 5'D-II is an similar t o 200-kDa multimeric protein containing a 29-kDa substrate-binding sub unit (p29) and an unknown number of other subunits, In the absence of thyroxine (T-4), p29 is slowly endocytosed and transported to the lyso somes, T-4 treatment rapidly activates an actin-mediated endocytotic p athway and targets the enzyme to the endosomes, In this study, we have characterized the influence of T-4 on the intracellular trafficking o f 5'D-II. We show that T-4 accelerates the rate of 5'D-II inactivation by translocating the enzyme to the interior of the cell and by seques tering p29 in the endosomal pool without accelerating the rate of degr adation of p29. This dichotomy between the rapid inactivation of catal ytic activity and the much slower degradation of p29 is consistent wit h the reuse of p29 in the production of 5'D-II activity, Immunocytoche mical analysis with a specific anti-p29 IgG shows that pulse affinity- labeled p29 reappears on the plasma membrane similar to 2 h after enzy me internalization in the presence of T-4, indicating that p29 is recy cled, Despite the ability of p29 to be recycled in the T-4-treated cel l, 5'D-II catalytic activity requires ongoing protein synthesis, presu mably of another enzyme component(s) or an accessory enzyme-related pr otein, In the absence of T-4, enzyme inactivation and p29 degradation are temporally linked, and pulse affinity-labeled p29 is internalized and sequestered in discrete intracellular pools, These data suggest th at T-4 regulates fundamental processes involved with the turnover of i ntegral membrane proteins and participates in regulating the inter-rel ationships between the degradation, recycling, and synthetic pathways.