Biology Reference
In-Depth Information
construct in cells displaces the cognate, endogenous, active, and sequestered form
from its site of action without affecting the activity of free populations of that PDE.
In so doing, local cAMP or cGMP concentration around the complex involving the
sequestered PDE will selectively rise, thereby generating a unique phenotypic
signature that mimics inhibition of the sequestered PDE. It will be of great interest
if either small molecules or peptidomimetics that disrupt specific PDE partnerships
can be identified for therapeutic advantage. Certainly proof of principal has been
garnered for this approach by the development of dominant negative constructs for
PDE4 isoforms as elucidated above (Lynch et al.
2005
) and use of cell-permeable
peptides that duplicate the binding surface of one partner and thus disrupt specific
PDE partnerships in intact cells (Murdoch et al.
2007
; Smith et al.
2007
).
The concept that PDEs interact with and are regulated by other proteins was first
discovered from studies of the PDE1 family, which is regulated by reversible
interaction with calmodulin, and the PDE6 family, which is located in photoreceptor
rod and cone cells and regulated by interaction with the small inhibitory P
g
proteins
and activated transducin. For other PDE families, appreciation of this type of
regulation is relatively new. Consequently, the focus in studying protein interactions
involving most PDEs has been devoted to identifying protein partnerships, defining
modes of interaction, and appreciating the functional significance of such interac-
tions. The paradigm for PDE partner proteins that profoundly regulate PDE catalytic
site functions is well illustrated by control of PDE6 cN-hydrolyzing activity through
the direct interaction of its catalytic subunits with its inhibitory P
g
-subunits and the
effect of transducin in the activated state to bind to PDE6
g
in that complex, thereby
relieving the PDE6
g
inhibitory effects (Bender and Beavo
2006
). Elegant biochemi-
cal and structural studies have recently allowed the molecular basis of this to be
determined (Barren et al.
2009
; Zhang et al.
2009
). Two distinct types of interactions
between P
g
and PDE6 catalytic subunits that provide for the potent inhibition of
PDE6 catalysis have been proposed. One set of interactions involves direct contact of
the carboxyl-terminal residues of P
g
with the PDE6 catalytic pocket, thereby block-
ing cGMP entry. The second set of interactions involves binding of other regions of
P
g
to the PDE6 catalytic subunit, so as to attenuate catalytic activity in an allosteric
manner. Thus, activation of PDE6 by the GTP-bound form of transducin apparently
requires interaction with the carboxyl-terminus of PDE6
g
as well as additional
regions of PDE6
g
to relieve the inhibitory constraint on the PDE6 catalytic subunits.
Proteins that interact with members of other PDE families may also employ the
strategy of multiple contacts that diversely influence enzymatic activity, specific
localization, and functional features; such complexities should always be considered.
The regulation of PDEs by protein-protein interactions has been explicitly and
elegantly addressed in studies performed with PDE4, where a large number of
binding partners have been identified (Houslay
2010
). It is already appreciated from
phosphorylation studies of PDE4 isoforms (described above) that conformational
changes induced by phosphorylation are associated with altered activity and sensi-
tivity to some inhibitors (Conti et al.
2003
; Houslay and Adams
2003
; Houslay et al.
2005
). Four PDE4 genes (A/B/C/D) encode around 25 isoforms that are character-
ized by isoform-specific amino-terminal regions. These are then grouped based
