Evidence of Phospho‐degron regulating expression of urate secretory transporter ABCG2

Abstract

ABCG2 is a high capacity urate secretory transporter of the renal proximal tubule. Mutations in ABCG2 cause gout in humans. The ABCG2 patient mutation ABCG2Q141K leads to instability of the ABCG2 nucleotide‐binding domain, which can be rescued by enhancing nucleotide‐binding domain sandwich formation with phosphomimetic amino acid substitutions (G188E, S187D); raising the possibility that phosphorylation of residues in the NBD domain may regulate expression and trafficking of ABCG2. An in silico analysis of ABCG2 revealed a limited number of predicted phosphorylation sites including S195, a serine conserved in the mammalian lineage. The upstream RXRXS represents a target motif for both AKT1 and PKA, and we find both co‐immunoprecipitate with ABCG2. Specifically, endogenous AKT1 can pull down both over expressed ABCG2 in HEK293 cells as well as endogenous ABCG2 in mouse kidney lysate. AKT1 and ABCG2 transcript co‐localize in the proximal S2 segment of the mammalian nephron (Mark Knepper RNA‐seq database) and inhibiting the AKT1 kinase cascade with PI3K inhibitor LY294002 dramatically up‐regulated ABCG2 expression. Conversely, activating the AKT1 cascade with FBS down‐regulated ABCG2 expression. Replacement of the S195 residue with the phosphomimetic aspartic acid results in significant reduction in expression, localization of ABCG2 protein to peri‐nuclear compartments, and significant sensitivity to the proteasome inhibitor MG132; marking the S195 residue as a Phospho‐degron. Phosphorylation of the well conserved S195 residue may interfere with ATP binding and/or nucleotide‐binding domain sandwich formation, suggesting a novel regulatory mechanism for function and trafficking in ABC transporters.Support or Funding InformationAmerican Heart Association: 14SDG18060004 & Ardea BioSciences