The Chaperone-like Properties of Mammalian Inhibitor-2 Are Conserved in a Drosophila Homologue,

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Abstract
Phosphatase inhibitor-2 (I-2) is a mammalian phosphoprotein that binds to the catalytic subunit of type 1 serine/threonine phosphoprotein phosphatase (PP1c) and inhibits its activity in vitro. Recombinant PP1c differs from native PP1c in several biochemical criteria, including the requirement for Mn(2+), sensitivity to vanadate, and p-nitrophenyl phosphate (pNPP) phosphatase activity. I-2 can convert recombinant PP1c into a native-like activity in vitro. It has therefore been suggested that I-2 may act as a molecular chaperone for PP1 in vivo. We have identified a Drosophila homologue (I-2Dm) in a two-hybrid screen for PP1c-binding proteins. The sequence of I-2Dm is 35% identical with that of I-2, whereas the catalytic subunits themselves are >85% identical in flies and humans; however, we show that many biochemical properties of I-2 are conserved. Like I-2, I-2Dm can convert recombinant PP1c to a native-like activity. This strongly suggests that this ability is an essential, conserved role of I-2 and I-2Dm.