Purification and characterization of transducin from capybara Hydrochoerus hydrochaeris

Polypeptides of ∼ 39, 36 and ≤ 14.4 kDa remained tightly bound to illuminated Hydrochoerus hydrochaeris retinal rod outer segment (ROS) membranes following extensive isotonic and hypotonic washes, and were specifically released in the presence of GTP. These results identified them as the α-, β-and γ-subunits of transducin (T). Once purified to homogeneity by anion-exchange chromatography, capybara T showed light-dependent β,γ-imido-guanosine 5'-triphosphate (GMPPNP) binding and GTPase activities in the presence of bovine rhodopsin, was recognized by anti-bovine T polyclonal antibodies, and was ADP-ribosylated by pertussis toxin. Capybara T bound GMPPNP with an apparent Kd of 18 nM, and the Scatchard and Hill plots revealed positive cooperativity for binding to photoactivated rhodopsin. Using a coupled enzymatic spectrophotometric assay, the initial velocity of its intrinsic light-dependent GTP hydrolytic capacity was calculated to be 0.1 mol of GTP hydrolyzed/min/mol of capybara T. Additionally, chromatography on ω-amino octylagarose of GTPγS-extracted capybara T demonstrated that this protein is composed by two functional units, the α-subunit and the βγ-complex. All these results showed that capybara T possesses similar characteristics to other reported transducins. Interestingly, T is the first protein involved in phototransduction that is purified and characterized from H. hydrochaeris.