JBC, Vol. 250, Issue 19, 7639-7646, Oct, 1975
Membrane-bound tubulin in brain and thyroid tissue
B. Bhattacharyya and J. Volff
Brain and thyroid tissue contain membrane-bound colchicine-binding activity
that is not due to contamination by loosely bound cytoplasmic tubulin. This
activity can be solubilized to the extent of 80 to 90% by treatment with
0.2% Nonidet P-40 with retention of colchicine binding. Extracts so
obtained contain a prominent protein band in disc gel electrophoresis that
co-migrates with tubulin. Membranes, and the solubilized protein therefrom,
exhibit ligand binding properties like tubulin; for colchicine the KA is
approximately 1 X 10(6) M-1 in brain and approximately 0.6 X 10(6) M-1 in
thyroid; for vinblastine the KA is approximately 8 X 10(6) M-1 for both
tissues; and for podophyllotoxin the Ki is approximately 2 X 10(-6) M for
both tissues. Displacement by analogues of colchicine is of the same order
as for soluble tubulin. Although membrane-bound colchicine-binding activity
shows greater thermal stability and a higher optimum binding temperature
(54 degrees versus 37 degrees) than soluble tubulin, this appears to be the
result of the membrane environment since the solubilized binding activity
behaves like the soluble tubulin. Antibody against soluble brain tubulin
reacts with membranes and solubulized colchicine-binding activity from both
brain and thyroid gland. We conclude that brain and thyroid membrane
preparations contain firmly bound tubulin or a very similar protein.