The Regulation of the cGMP-binding cGMP Phosphodiesterase by Proteins That Are Immunologically Related to γ Subunit of the Photoreceptor cGMP Phosphodiesterase

Abstract

The cGMP phosphodiesterase from retinal rods (PDE-6) is an ␣␤␥ 2 heterotetramer. The ␣ and ␤ subunits contain catalytic sites for cGMP hydrolysis, whereas the ␥ subunits serve as a protein inhibitor of the enzyme. Visual excitation of photoreceptors enables the activated GTP-bound form of the G-protein transducin to remove the inhibitory action of the ␥ subunit, thereby triggering PDE-6 activation. The type 5 phosphodiesterase (PDE-5) isoform shares a number of similar characteristics with PDE-6, including binding of cGMP to noncatalytic sites, the cyclic nucleotide specificity, and inhibitor sensitivities. Although the functional role of PDE-5 remains unclear, it has been shown to be activated by protein kinase A (PKA) (Burns, F., Rodger, I. W. & Pyne, N. J. (1992) Biochem. J. 283, 487-491). Here we report that both the recombinant ␥ subunit and a peptide corresponding to amino acids 24 -46 in this protein inhibited the activation of PDE-5 by PKA. Furthermore, immunoblotting airway smooth muscle membranes with a specific antibody against amino acids 24 -46 of the PDE-6 ␥ subunit identified two major immunoreactive small molecular mass proteins of 14 and 18 kDa (p14 and p18). These appear to form a complex with PDE-5, because PDE activity was immunoprecipitated using antibody against the PDE-6 ␥ subunit. p14 and p18 were also substrates for phosphorylation by a unidentified kinase that was stimulated by a pertussis toxin-sensitive Gprotein. Phosphorylation of p14/p18 in membranes treated with guanine nucleotides correlated with a concurrent reduction in the activation of PDE-5 by PKA. We suggest that p14 and p18 share an epitope common to PDE-6 ␥ and that this region may interact with PDE-5 to prevent its activation by PKA.

Novel Alternative Splice Variants of cGMP-binding cGMP-specific Phosphodiesterase

“…Photoreceptor PDEs (PDE6), which are homologous to cGB-PDE, are known as membrane-associated proteins and are inhibited by ␥ subunits of PDE6 on retinal membrane (5). The PDE6 ␥ subunit is also noted to prevent the activation of cGB-PDE by cAMP-dependent protein kinase (42). It was reported that a form of the PDE4 family, named RNPDE4A5, interacted with SH3 domain of Src tyrosyl protein kinase, and that its activity is regulated by such proteins (28).…”

Characteristics of Photoreceptor PDE (PDE6): similarities and differences to PDE5

“…20,35,36 The g subunit of PDE6 also acts as a substrate for phosphorylation at several distinct sites within the central region of this 10 kDa protein. Phosphorylation of g at Thr (22) or Thr (35) has little effect on the PDE6 holoenzyme itself, but greatly diminishes the ability of activated transducin to bind the g subunit and relieve inhibition of catalysis. 37 Phosphorylation of g at Thr(62) in nonretinal tissue has been reported to regulate mitogenic signaling via interactions with proteins other than PDE6 catalytic subunits (whose expression is confined to the retina and pineal gland).…”

“…3 For PDE5, enzyme activation most likely proceeds via phosphorylation of the catalytic subunits and allosteric changes in cGMP binding to the GAF domains. 20,21 (While PDE5 has been reported to copurify with the PDE6 g subunit, 22 the physiological significance is uncertain because the g subunit lacks binding or inhibitory activity toward PDE5 in vitro. 23 )…”

Cyclic GMP Phosphodiesterase-5: Target of Sildenafil

“…Burns et al (57) have reported that a partially purified PDE5 from guinea pig lung is activated when phosphorylated by PKA. PDE5 may also be regulated by other low molecular weight factors, and these could alter the effects of phosphorylation (58). As is the case for PDE4, PDE5 may also be subject to long term regulation through changes in enzyme concentration in some cell types (59 -61).…”