Protein kinase A (PKA; OMIM 176911) mediates actions of hormones and neurotransmitters that activate adenylate cyclase
. Signals carried by cAMP are often directed at discrete intracellular sites. A nonuniform distribution of PKA
type II molecules occurs when they are attached to the cytoskeleton by 'A-kinase anchor proteins' .
Such anchored molecules may be essential for dissemination of cAMP signals in highly polarized epithelium such as lung
and kidney.
General function
Comment
Cellular localization
Cytoplasmic, Cytoskeleton
Comment
Ovarian function
Follicle development
Comment
Carr DW, 1999 reported the identification of cAMP-dependent protein kinase (PKA) holoenzymes in
preantral- and preovulatory-follicle-enriched ovaries, and their
association with A-kinase-anchoring proteins (AKAP).
PKA activity, PKA holoenzymes, PKA subunits and AKAPs from preantral (PA) follicles and
preovulatory (PO) follicles ovaries were compared. Soluble PKA holoenzymes and regulatory (R) subunits
were separated by DEAE-cellulose chromatography and sucrose-density-gradient
centrifugation. PKA R subunits were distinguished by photoaffinity labelling,
autophosphorylation, size, isoelectric point and immunoreactivity. AKAPs were
identified by RII subunit overlay assays and immunoreactivity. The results showed
that extracts from PA and PO ovaries exhibited equivalent PKA holoenzyme profiles
and activities, characterized by low levels of PKA type I (PKAI) holoenzyme and two
distinct PKAII holoenzyme peaks, one containing only RIIbeta subunits (PKAIIbeta)
and one containing both PKAIIbeta and PKAIIalpha holoenzymes. Both PA and PO
ovarian extracts also contained PKA catalytic (C)-subunit-free RIalpha, while only
PO ovaries exhibited C-subunit-free RIIbeta. Consistent with the elevated levels of
C-subunit-free RIIbeta in PO cells, PKA activation in PO cells required higher
concentrations of forskolin than that in PA cells. While extracts of PA and PO ovaries
exhibited a number of similar AKAPs, including four prominent ones reactive with
anti-AKAP-KL antisera (where AKAP-KL is an AKAP especially abundant in kidney
and liver), cAMP-agarose affinity chromatography revealed two major differences in
AKAP binding to purified R subunits. PO ovaries contained increased levels of
AKAP80 (AKAP of 80 kDa) bound selectively to R subunits in DEAE-cellulose peak
2 (comprising PKAIIbeta and RIalpha), but not to R subunits in DEAE-cellulose peak
3 (comprising PKAIIalpha, PKAIIbeta and RIIbeta). PO ovaries also showed
increased binding of R subunits to AKAPs reactive with anti-AKAP-KL antisera at
210, 175, 150 and 115 kDa. Thus in PO ovaries, unlike in PA ovaries, the majority of
AKAPs are bound to R subunits. These results suggest that altered PKA-AKAP
interactions may contribute to the distinct responses of PA and PO follicles to high
levels of cAMP, and that higher cAMP levels are required to activate PKA in PO
ovaries.
Expression regulated by
FSH
Comment
Carr DW,et al reported the Follicle-stimulating hormone regulation of A-kinase anchoring
proteins in granulosa cells.
They show that FSH action alters
the expression of A-Kinase Anchoring Proteins (AKAPs), which function to target the
subcellular distribution of the type II PKA. Exposure of granulosa cells grown in
primary culture with FSH and estradiol for 72 h resulted in the up-regulation of an
80-kDa AKAP and the RII beta subunit of PKA, whereas cells grown in control
medium containing only estradiol produced a time-dependent increase of a 140-kDa
AKAP. RII overlays performed with [32P]RII alpha preferentially detected
RII-binding bands of 80 and 95 kDa compared to blots probed with [32P]RII beta,
suggesting that FSH may alter the subcellular location of PKA in an isoform-specific
manner. FSH treatment causes a translocation of RII alpha from the particulate to the
cytosolic fraction coincident with the induction of the 80-kDa AKAP, which is also
predominately cytosolic. These data suggest that FSH promotes a redistribution of the
type II PKA holoenzyme through the selective induction of an RII isoform-specific
AKAP.