The β subunits of heterotrimeric G-protein in thyroid plasma membranes
Grazyna Adler, Urszula Piotrowska & Ireneusz Kozicki
TSH regulates thyroid function trough the receptor coupled to heterotrimeric G-protein composed of α, β and gamma subunits. The activation of α s /adenylyl cyclase mediated pathway accounts for most of TSH biological effects. TSH can also induce the other G-α protein mediated activities. Five different isoforms of β subunit have been described and it can be assumed that specific α-β combination are essential for functional differentiation of G proteins.
The present work was done to characterize isoforms of the β subunits of thyroid G-protein.
Triton X100 solubilized membrane fraction from cultured thyroid cells and postoperative thyroids from Graves disease and struma nodosa patients were studied. Soluble membrane proteins were characterized by immunoblotting with specific anti-β subunits antibodies. Additionally the mRNA from cells and thyroid homogenates were separated, translated into DNA, amplified with β-5 starters and electrophoreticaly characterized.
In plasma membrane fractions from cultured thyroid cells and thyroid tissues the β-1, β-2, β-3 and β-4 subunits, 35 kDa, were detected. Anti β-5 antibodies recognized 70 kDa protein with probably corresponded to β-5 dimmer or its complex with other protein. Both in cells and tissue homogenates the β-5 mRNA was found, which also supports the presence of β-5 isoform in thyroid. In membranes of cells and different thyroids the amount of particular β isoform differs. To estimate the relative amount of isoforms the concurrence in specific antibodies binding between pooled thyroids and individual thyroids were studied. It was found that the main β isoform of G-protein in thyroid membranes is β-2. We assume that in thyroid the β-2 is the isoform cooperating with the α subunit of Gs-protein.