Brain protein phosphatase 2A: Developmental regulation and distinct cellular and subcellular localization by B subunits

Abstract

Protein phosphatase 2A (PP2A) is a heterotrimeric enzyme consisting of a catalytic subunit (C), a structural subunit (A), and a variable regulatory subunit (B). We have investigated the spatial and temporal expression patterns of three members of the B subunit family, Bα, Bβ, and Bγ, both at the message level by using ribonuclease protection analysis and at the protein level by using specific antibodies. Although A, Bα, and C protein are expressed in many tissues, Bβ and Bγ were detectable only in brain. Bα, Bβ, and Bγ are components of the brain PP2A heterotrimer, because they copurified with A and C subunits on immobilized microcystin. Whereas B, and Bβ are mainly cytosolic, Bγ is enriched in the cytoskeletal fraction. In contrast to A, C, and Bα, which are expressed at constant levels, Bβ and Bγ RNA and protein are developmentally regulated, with Bβ levels decreasing and Bγ levels increasing sharply after birth. RNA and immunoblot analyses of subdissected brain regions as well as immunohistochemistry demonstrated that B subunits are expressed in distinct but overlapping neuronal populations and cellular domains. These data indicate that B subunits confer tissue and cell specificity, subcellular localization, and developmental regulation to the PP2A holoenzyme. The Bα-containing heterotrimer may be important in general neuronal functions that involve its partially nuclear localization. Holoenzymes containing Bβ likely function in early brain development as well as in somata and processes of subsets of mature neurons. Bγ may target PP2A to cytoskeletal substrates that are important in the establishment and maintenance of neuronal connections.