The structure of Tap42/alpha4 reveals a tetratricopeptide repeat-like fold and provides insights into PP2A regulation

Physiological functions of protein phosphatase 2A (PP2A) are determined via the association of its catalytic subunit (PP2Ac) with diverse regulatory subunits. The predominant form of PP2Ac assembles into a heterotrimer comprising the scaffolding PR65/A subunit together with a variable regulatory B subunit. A distinct population of PP2Ac associates with the Tap42/alpha4 subunit, an interaction mutually exclusive with that of PR65/A. Tap42/alpha4 is also an interacting subunit of the PP2Ac-related phosphatases, PP4 and PP6. Tap42/alpha4, an essential protein in yeast and suppressor of apoptosis in mammals, contributes to critical cellular functions including the Tor signaling pathway. Here, we describe the crystal structure of the PP2Ac-interaction domain of Saccharomyces cerevisiae Tap42. The structure reveals an all alpha-helical protein with striking similarity to 14-3-3 and tetratricopeptide repeat (TPR) proteins. Mutational analyses of structurally conserved regions of Tap42/alpha4 identified a positively charged region critical for its interactions with PP2Ac. We propose a scaffolding function for Tap42/alpha4 whereby the interaction of PP2Ac at its N-terminus promotes the dephosphorylation of substrates recruited to the C-terminal region of the molecule.