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 protein kinase A (PKA). ARPP‐16 is enriched in dopamine‐responsive medium spiny neurons in the striatum, while ARPP‐19 is ubiquitously expressed. ARPP‐19 is highly homologous to α‐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 show that ARPP‐19 is composed of at least two differentially expressed isoforms (termed ARPP‐19 and ARPP‐19e/endosulfine). All ARPP‐16/19 family members 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 the phosphorylated form of ARPP‐16/19/19e was used to examine the phosphorylation of ARPP‐16/19 family members in intact cells. In striatal slices, the phosphorylation of ARPP‐16 was increased in response to activation of D1‐type dopamine receptors, and decreased in response to activation of D2‐type dopamine receptors. In non‐neuronal cells, ARPP‐19 was highly phosphorylated in response to activation of PKA. These results establish that ARPP‐16/19 proteins constitute a family of PKA‐dependent intracellular messengers that function in all cells. The high levels of ARPP‐16 in striatal neurons and its bi‐directional regulation by dopamine suggest a specific role in dopamine‐dependent signal transduction. The conservation of this protein family through evolution suggests that it subserves an important cellular function that is regulated by PKA.