Structure of PDE4

The mammalian PDE4 family comprises around 20 different isoforms encoded by four genes, PDE4A, PDE4B, PDE4C and PDE4D (Conti et al. 2003; Houslay and Adams 2003; Houslay et al. 2005) (Fig. 2). Each gene is large and complex, spanning around 50 kb and having around 20 exons, which gives rise to multiple isoforms by alternative mRNA splicing (Monaco et al. 1994; Bolger et al. 1997). Each isoform has a modular structure consisting of an isoform-specific N-terminal region, all or part of

Pde Structure Schematic

Fig. 2 The PDE4 enzyme family. Schematic diagram of the gene organization of the four PDE4 sub-families showing the exons that encode the core UCR1/2 and catalytic regions, the exons that encode the long form 5' unique regions and the positions of other unique exons. Also shown are the four isoform subcategories based upon presence or absence of UCR1/2 regions and generated by alternative mRNA splicing

Fig. 2 The PDE4 enzyme family. Schematic diagram of the gene organization of the four PDE4 sub-families showing the exons that encode the core UCR1/2 and catalytic regions, the exons that encode the long form 5' unique regions and the positions of other unique exons. Also shown are the four isoform subcategories based upon presence or absence of UCR1/2 regions and generated by alternative mRNA splicing two regulatory sequences termed upstream conserved region 1 (UCR1) and UCR2, a highly conserved catalytic domain and a sub-family-specific C-terminal region (Bolger et al. 1993) (Fig. 2). The unique N-terminal region is generally encoded by a single exon, except for the first or most 5' isoform of each gene, which seems to be encoded by two or more 5' exons (Sullivan et al. 1999; Rena et al. 2001; D'Sa et al. 2002; Wang et al. 2003), and which is immediately downstream of the promoter that drives its expression (Rena et al. 2001; Wallace et al. 2005). This unique N-terminal region characterizes each isoform and confers the ability to interact with specific binding partners and thus be targeted to distinct intracellular regions (see Sects. 2.4 and 2.5). UCR1 and UCR2 are each encoded by three separate exons and are highly conserved between the four PDE4 sub-families (Bolger 1994). These regulatory regions are unique to isoforms of the PDE4 family, and their presence, in conjunction with susceptibility to rolipram inhibition, distinguishes PDE4s from all other classes of PDE. The various PDE4 isoforms can be further categorized into four groups, namely the long PDE4 forms that contain both UCR1 and UCR2, the short forms that lack UCR1, but have an intact UCR2, the super-short forms that not only lack UCR1, but have an N-terminally truncated UCR2 (Houslay 2001), and the "dead-short" isoforms that are both N- and C-terminally truncated so as to be catalytically inactive (Johnston et al. 2004; Houslay et al. 2007). UCR1 and UCR2 function as a regulatory domain that controls the catalytic unit and confers regulatory functions on PDE4 by orchestrating the functional outcome of phosphorylation by PKA and ERK (Houslay and Adams 2003). UCR2 is joined to UCR1 by linker region 1 (LR1) and to the catalytic region by LR2. These short linker regions vary greatly between PDE4 sub-families for reasons that are not understood at present. The catalytic unit is encoded by six exons and, as might be expected, is highly conserved between members of the PDE4 family. Solving of the crystal structure of PDE4B2 catalytic unit revealed that it comprises three sub-domains formed by 17 a-helices (Xu et al. 2000; Ke and Wang 2007). The cAMP-binding active site is created as these three sub-domains come together to form a deep pocket containing two metal ions essential for catalytic activity and lined with hydrophobic and negatively charged residues, The structure, proposed catalytic mechanism and conformational changes of the catalytic unit have been discussed in detail elsewhere (Houslay and Adams 2003; Houslay et al. 2005).

The final exon encodes part of the catalytic unit together with the sub-family-specific C-terminal region, which is of unknown function, but which has been used advantageously to generate sub-family-specific antisera (Houslay et al. 1998).

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