Biology Reference
In-Depth Information
2.4
Interaction with Other Proteins
For many years, PDE activity in cells was referred to either as soluble or particulate, but
in most cases, there has been little understanding of either the importance or
the molecular basis for this partitioning. In fact, members of the same PDE families
could be found in both the cytosolic and the particulate fractions. Such complexes occur
both at particular cell membranes and within the cytosolic fraction. More recently, it
has become apparent that one reason for the diversity of PDEs in most tissues is to
provide for their selective interaction with other proteins in signaling complexes
allowing dynamic regulation of particular signaling processes and thus giving rise to
the concept of “targeted cAMP degradation” (Houslay
2001
,
2010
). Indeed, it was
initial studies in cardiac myocytes that provided a major building block for our
understanding of compartmentalized cAMP signaling and its mechanistic implications
(Corbin et al.
1977
; Hayes et al.
1979
; Steinberg and Brunton
2001
; Zaccolo and
Pozzan
2002
).
Selective and diverse compartmentalization of a certain portion of the isoforms
within a PDE family has now been well described for PDEs 1, 2, 3, 4, 5, 6, and 10.
It seems highly likely that interaction of these PDEs with other proteins to form
signaling complexes will affect inhibitor potencies. Indeed, this has been clearly
demonstrated for various PDE4 partnerships, such as PDE4A4 with various SH3
domain containing proteins (McPhee 1999); PDE4D5 with RACK1 (Yarwood et al.
1999
) and PDE4A4 with the aryl hydrocarbon interacting protein (AIP) (Bolger
et al.
2003
), where recent structural studies have afforded a potential molecular
explanation for such actions (Burgin et al.
2010
).
In addition, the determinants that dictate the proportion of a PDE family that is
localized to particular intracellular domains or “free” in the cytosol are only begin-
ning to be understood (Houslay
2010
) and include competition by different scaffolds
for binding specific PDEs and dynamic changes in association patterns that are
triggered by phosphorylation (Collins et al.
2008
; Murdoch et al.
2007
) and ubiqui-
tination (Li et al.
2009
). Moreover, when “free” PDEs are translocated from the
cytosol to a more restricted and defined locale, the impact on the cN metabolism in
the cellular territory that was vacated has yet to be formally considered. Are there
“pools/banks” of PDEs that are simply not needed in normal cellular functioning and
await a call for action as needed in particular locales in the cell? Are these meaning-
ful pools of PDE activity that should/could be targeted by inhibitors? The very
different actions of inhibitors selective for different cAMP-hydrolyzing PDEs, such
as PDEs1/2/3/4, were originally interpreted as reflecting actions related to the
relative abundance of such enzymes in cells. However, it is now clear that function-
ally distinct pools of cAMP regulated by targeted PDEs is critical in controlling key
cellular processes in cells. The most conclusive proof for a specific mechanism of
subcellular targeting comes from studies on PDE4s (Houslay
2001
,
2010
). Critical to
this concept was the discovery that PDE4A1, an enzyme that is entirely membrane
bound, is generated by splicing a membrane-targeting domain located within its
isoform-specific amino-terminal amino acid sequence to a segment of sequence
