Purification and Subunit Structure of Rat‐Liver Phosphoprotein Phosphatase whose Molecular Weight is 260000 by Gel Filtration (Phosphatase IB)

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Abstract
Phosphoprotein phosphatase IB is a form of rat liver phosphoprotein phosphatase, distinguished from the previously studied phosphoprotein phosphatase II by earlier elution from DEAE-cellulose, by higher MW on gel filtration (260,000) and by lower activity toward phosphorylase a. This enzyme was purified to apparent homogeneity by chromatography on DEAE-cellulose, aminohexyl-Sepharose-4B, histone-Sepharose-4B, protamine-Sepharose-4B and Sephadex G-200. The MW of purified phosphatase IB was 260,000 by gel filtration and 185,000 from s20,w and Stokes'' radius. Using histone phosphatase activity as the reference for comparison, the phosphorylase phosphatase activity of purified phosphatase IB was only one-fifth that of phosphatase II. Sodium dodecyl sulfate gel electrophoresis revealed that phosphatase IB contains 3 types of subunit, i.e., .alpha., .beta. and .gamma., whose MW are 35,000, 69,000 and 58,000, respectively. The .alpha. subunit is identical to the .alpha. subunit of phosphatase II. While the .beta. subunit is also identical or similar to the .beta. subunit of phosphatase II, the .gamma. subunit appears to be unique to phosphatase IB. When purified phosphatase IB was treated with 2-mercaptoethanol at -20.degree. C, the enzyme was dissociated to release the catalytically active .alpha. subunit. Along with this dissociation, there was a 7.4-fold increase in phosphorylase phosphatase activity; but histone phosphatase activity increased only 1.6-fold. The possible functions of the .gamma. subunit are discussed in relation to this activation of enzyme.

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