ARPP-16/ARPP-19: a highly conserved family of cAMP-regulated phosphoproteins

ARPP-16 and ARPP-19 are closely related cAMP-regulated phosphoproteins that were initially discovered in mammalian brain as in vitro substrates for pr otein kinase A (PKA). ARPP-16 is enriched in dopamine-responsive medium spi ny neurons in the striatum, while ARPP-19 is ubiquitously expressed. ARPP-1 9 is highly homologous to alpha -endosulfine and database searches allowed the identification of novel related proteins in D. melanogaster, C. elegans , S. mansoni and yeast genomes. Using isoform-specific antibodies, we now s how that ARPP-19 is composed of at least two differentially expressed isofo rms (termed ARPP-19 and ARPP-19e/endosulfine). All ARPP-16/19 family member s contain a conserved consensus site for phosphorylation by PKA (RKPSLVA in mammalian ARPP-16 and ARPP-19), and this site was shown to be efficiently phosphorylated in vitro by PKA. An antibody that specifically recognized th e phosphorylated form of ARPP-16/19/19e was used to examine the phosphoryla tion of ARPP-16/19 family members in intact cells. In striatal slices, the phosphorylation of ARPP-16 was increased in response to activation of D-1-t ype dopamine receptors, and decreased in response to activation of D-2-type dopamine receptors. In non-neuronal cells, ARPP-19 was highly phosphorylat ed in response to activation of PKA. These results establish that ARPP-16/1 9 proteins constitute a family of PKA-dependent intracellular messengers th at function in all cells. The high levels of ARPP-16 in striatal neurons an d its bi-directional regulation by dopamine suggest a specific role in dopa mine-dependent signal transduction. The conservation of this protein family through evolution suggests that it subserves an important cellular functio n that is regulated by PKA.