Mechanism for the regulation of mammalian cGMP phosphodiesterase6. 1: Identification of its inhibitory subunit complexes and their roles
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- Yamazaki, A., Bondarenko, V.A., Matsuura, I. et al. Mol Cell Biochem (2010) 339: 215. doi:10.1007/s11010-010-0387-8
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Cyclic GMP phosphodiesterase (PDE) in bovine rod photoreceptor outer segments (OS) comprises a catalytic subunit complex (Pαβ) and two inhibitory subunits (Pγ) and is regulated by the α subunit of transducin (Tα). Here, we show an overall mechanism for PDE regulation by identifying Pγ complexes in OS homogenates prepared with an isotonic buffer. Before Tα activation, three Pγ complexes exist in the soluble fraction. Complex a, a minor complex, contains Pαβ, Tα, and a protein named Pδ. Complex b, Pαβγγb, has a PDE activity similar to that of membranous Pαβγγ, PαβγγM, and its level, although its large portion is Pδ-free, is estimated to be 20–30% of the total Pαβγγ. Complex c, (Pγ·GDP-Tα)2c, appears to be a dimer of Pγ·GDP-Tα. Upon Tα activation, (1) complex a stays unchanged, (2) Pαβγγb binds to membranes, (3) the level of (Pγ·GDP-Tα)2c is reduced as its GTP-form is produced, (4) complex d, Pγ·GTP-Tαd, is formed on membranes and its substantial amount is released to the soluble fraction, and (5) membranous Pαβγγ, PαβγγM and/or Pαβγγb, becomes Pγ-depleted. These observations indicate that Pγ as a complex with GTP-Tα dissociates from Pαβγγ on membranes and is released to the soluble fraction and that Pγ-depleted PDE is the GTP-Tα-activated PDE. After GTP hydrolysis, both (Pγ·GDP-Tα)2c and Pγ·GDP-Tαd, without liberating Pγ, deactivate Pγ-depleted PDE. The preferential order to be used for the deactivation is membranous Pγ·GDP-Tαd, solubilized Pγ·GDP-Tαd and (Pγ·GDP-Tα)2c. Release of Pγ·GTP-Tα complexes to the soluble fraction is relevant to light adaptation.