Mitogenic stimulation of cells by extracellular factors is often mediated by transmembrane tyrosine kinase receptors or receptors that associate with cytoplasmic tyrosine kinases. The signaling pathways generated by many of these receptors are well characterized (23
). In contrast to the tyrosine kinase receptors, the receptor signaling pathways for transforming growth factor β (TGF-β) and the many TGF-β-related factors have only recently been characterized (12
). TGF-β and TGF-β-related factors are secreted proteins which mediate their activities through transmembrane serine/threonine kinase receptors. Ligand-induced activation of these receptors and signaling leads to potent growth inhibition and gene expression responses. Two type I and two type II receptors form the signaling TGF-β receptor complex at the cell surface, in which the type II receptors (TβRII) are constitutively active and autophosphorylated, and the type I receptors (TβRI) require phosphorylation by TβRII for activation (12
Several proteins have been shown to associate with TGF-β receptors. Smad2 and Smad3, which act as effectors of TGF-β signaling, can associate with the receptor complex and are phosphorylated by TβRI. Once dissociated, they are translocated as a complex with Smad4/DPC4 into the nucleus, where they function as transcriptional activators (11
). Another receptor-associated protein is TRIP-1, which interacts with and is phosphorylated by TβRII (8
) and contains five WD-40 repeats (40
). WD-40 repeats are minimally conserved sequences of approximately 40 amino acids that typically end in tryptophan-aspartate (WD) and are thought to mediate protein-protein interactions (40
). Since TRIP-1 is largely composed of WD-40 repeats, it is possible that other WD-40 repeat proteins may bind to serine/threonine kinase receptors. The association of WD-40 repeat proteins may then allow them to play a role in signaling by the serine/threonine kinase receptors. WD-40 repeats have been identified in a variety of proteins (40
), including the Bα subunit of the serine/threonine protein phosphatase 2A (PP2A).
PP2A is one of the major, albeit poorly understood, serine/threonine phosphatases which regulates several processes, including signaling (43
) and cell cycle progression (9
). PP2A exists as a dimeric core of a catalytic (C) and a structural (A) subunit or as a trimeric complex with a regulatory subunit (B), of which there are several forms. Bα and Bβ contain five WD-40 repeats (40
), whereas B′/B56 (38
) and B" (46
) are structurally unrelated and lack WD-40 repeats (45
). Bα regulates the catalytic activity of PP2A, and this activity has been implicated in cell cycle control (21
). The differential interactions of these regulatory B subunits with the AC core enzyme suggest a complex pattern of regulation, which may explain the various functions of PP2A in growth control.
Because Bα has been implicated in cell cycle control (9
) and has WD-40 repeats like TRIP-1 does, we analyzed the physical and functional interactions between Bα and TGF-β receptors. In this report, we demonstrate that the Bα regulatory subunit of PP2A interacts with the cytoplasmic domains of type I TGF-β receptors and is a direct target for their kinase activity. The growth inhibitory activity of Bα is regulated by TGF-β receptors and cooperates with the direct antiproliferative effect of the TGF-β receptors. Thus, the association of the WD-40 repeat Bα subunit of PP2A with serine/threonine kinase receptors results in a functional interaction of TGF-β receptor signaling with Bα and may be important for our understanding of how PP2A activity is regulated.