Abstract
The beta-adrenergic receptor kinase (beta ARK) specifically phosphorylates
the agonist-occupied form of the beta-adrenergic and related G protein-coupled
receptors. Structural features of this enzyme have been elucidated
recently by the isolation of a cDNA that encodes bovine beta ARK.
Utilizing a catalytic domain fragment of the beta ARK cDNA to screen
a bovine brain cDNA library we have isolated a clone encoding a beta
ARK-related enzyme which we have termed beta ARK2. Overall, this
enzyme has 85% amino acid identity with beta ARK, with the protein
kinase catalytic domain having 95% identity. The ability of beta
ARK2 to phosphorylate various substrates was studied after expression
in COS 7 cells. Although beta ARK2 is essentially equiactive with
beta ARK in phosphorylating an acid-rich synthetic model peptide
it was only approximately 50% as active when the substrate was the
agonist-occupied beta 2-adrenergic receptor and only approximately
20% as active toward light-bleached rhodopsin. As with beta ARK,
phosphorylation of the receptor substrates by beta ARK2 was completely
stimulus dependent. RNA blot analysis with selected bovine tissues
reveals an mRNA of 8 kilobases with a distribution similar to that
of beta ARK. More detailed RNA analysis using a ribonuclease protection
assay in various rat tissues suggests that the beta ARK2 message
is present at much lower levels (typically 10-20%) than the beta
ARK message. In the rat the beta ARK2 mRNA is localized predominantly
in neuronal tissues although low levels are also observed in various
peripheral tissues. The beta ARK2 gene has been localized to a region
of mouse chromosome 5 whereas the beta ARK gene is localized on mouse
chromosome 19. These data suggest the existence of a "family" of
receptor kinases which may serve broadly to regulate receptor function.
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