Protein phosphatase 6 (PP6) is an essential Ser/Thr
phosphatase conserved among eukaryotes. The
Saccharomyces cerevisiae homologue of PP6 called Sit4 depends on association with SAPS domain
subunits. This study used a human SAPS domain
subunit FLAG-PP6R1 to identify endogenous interacting proteins.
Mass spectrometry identified coprecipitating proteins as PP6
catalytic subunit and three
ankyrin repeat proteins (Ankrd28, Ankrd44, and Ankrd52). These proteins have extensive
sequence identity to one another but segregate into separate branches on a
phylogenetic tree for vertebrate species, suggesting individual biological functions. Tagged Ankrd28 coprecipitated with PP6, not with
PP2A or PP4, and with SAPS domain
subunits PP6R1 and PP6R3. Tagged PP6 coprecipitated endogenous SAPS domain
subunits and Ankrd28. The
C-terminal region of PP6R1 was sufficient to coprecipitate Ankrd28, but not PP6, demonstrating that PP6R1 acts as a scaffold with separate regions for binding to PP6 and to Ankrd28. Endogenous PP6
holoenzymes with PP6R1 and PP6R3
subunits were resolved by
DEAE chromatography and eluted together with Ankrd28 at Mr > 440 kDa from
Superose 12.
Knockdown of PP6R1 or Ankrd28, but not PP6R3, produced equivalent enhancement of IkappaBepsilon degradation in response to TNFalpha. The results suggest that PP6 functions as a heterotrimer, composed of the PP6
catalytic subunit bound to a SAPS domain scaffold
subunit that associates with Ankrd28. We propose that the SAPS and
ankyrin repeat regulatory
subunits determine the function and specificity of PP6.
