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containing the regulatory and catalytic domains, that on its own is fully soluble, fully
active, and common to all members of the PDE4A subfamily (Baillie et al. 2002 ;
Huston et al. 2006 ; Scotland and Houslay 1995 ; Shakur et al. 1993 ; Smith et al.
1996 ).
Targeting of PDEs to particular regions of the cell is often dependent on
interaction with other proteins. Myriad proteins have been identified that interact
with the PDE4 family as a whole by binding to sites within core regions that are
conserved across the entire family or are specific for particular PDE4s by binding
to regions that are only common within specific subfamilies (Houslay 2001 , 2010 ;
Houslay et al. 2007 ). Such targeting allows particular PDE4 isoforms to associate
with specific signaling complexes and control the local cAMP level. This targeting
of specific PDEs underpins compartmentalized cAMP signaling, controlling the
threshold and persistence for activation of PKA and EPAC, for example, within
the environs of the complex containing the particular PDE that is sequestered there
(see e.g., Huston et al. 2008 ). Indeed, the use of spatially constrained, genetically
encoded cAMP reporters has allowed defined “pools” of cAMP to be detected and
the role of certain sequestered PDEs to be shown as pivotal in establishing and
maintaining cN gradients in cells (Mongillo et al. 2004 , 2006 ; Penmatsa et al. 2010 ;
Rich et al. 2001a , b , 2006 ; Zaccolo and Pozzan 2002 ).
PDE3A has also been shown to specifically form complexes with a variety of
proteins including 14-3-3 proteins, plectin, brefeldin A-inhibited guanine nucleo-
tide exchange proteins, and the cystic fibrosis transmembrane conductance regula-
tor channel (CFTR); in some instances, these localizations have also been shown to
control local pools of cAMP (Barnes et al. 2005 ; Mongillo et al. 2004 ; Penmatsa
et al. 2010 ; Puxeddu et al. 2009 ; Tasken et al. 2001 ). Likewise, a portion of platelet
PDE5 has been shown to form a complex with PKG and be selectively activated by
elevation of cGMP, thereby regulating both local cGMP level and calcium transi-
ents (Wilson et al. 2008 ). The presence of PDE5 in cardiomyocytes is controversial
(Lukowski et al. 2010 ; Vandeput et al. 2009 ). However, some investigators report it
to be present at the z-bands along with PKG, and their results suggest that this
localization of PDE5 is required for the antihypertrophic effects of sildenafil in the
heart (Kass et al. 2007 ; Nagayama et al. 2008 ; Takimoto et al. 2005a , b ). PDE7A1
and the C subunit of PKA have been shown to interact with high affinity
(K D ~ 0.5 nM), thereby blocking the catalytic activity of the C subunit, but the
effect of this interaction on catalytic function/inhibitor sensitivity of the PDE is not
known (Han et al. 2006 ).
Targeting a specific PDE isoform to a particular intracellular locale can be
expected to confer a functional role on that isoform that is inherently associated
with its unique spatial sequestration (Houslay 2010 ). This property cannot be
gauged by the use of selective inhibitors, genetic ablation (Jin et al. 1999 ), or
siRNA knockdown (Lynch et al. 2005 ) since each of these approaches will target
both the sequestered and free populations of that PDE. Instead, the use of dominant-
negative constructs has been successfully exploited to address this problem (Baillie
et al. 2003 ; Lynch et al. 2005 ; McCahill et al. 2005 ). In this approach, a single point
mutation that ablates catalytic activity while retaining overall structural integrity of
the PDE is engineered. Overexpression of such a catalytically inactive PDE
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