The C. elegans ASPP homolog APE-1 is a junctional protein phosphatase 1 modulator

How serine/threonine phosphatases are spatially and temporally tuned by regulatory subunits is a fundamental question in cell biology. Ankyrin repeat, SH3 domain, proline-rich-region-containing proteins (ASPPs) are protein phosphatase 1 (PP1) binding partners associated with cardiocutaneous diseases. ASPPs localize PP1 to cell-cell junctions, but how ASPPs localize and whether they regulate PP1 activity in vivo is unclear. Through a C. elegans genetic screen, we find that loss of the ASPP homolog, APE-1, suppresses a pathology called jowls, providing us with an in vivo assay for APE-1 activity. Using structure-function analysis, we discover that APE-1s N-terminal half directs the APE-1-PP1 complex to intercellular junctions. Additionally, we isolated mutations in highly conserved residues of APE-1s ankyrin repeats that suppress jowls yet do not preclude PP1 binding, implying ASPPs do more than simply localize PP1. Indeed, in vivo reconstitution of APE-1 suggests the ankyrin repeats modulate phosphatase output, a function we find to be conserved among vertebrate homologs.