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The E3 ubiquitin ligase- and protein phosphatase 2A (PP2A)-binding domains of the Alpha4 protein are both required for Alpha4 to inhibit PP2A degradation
Authors:LeNoue-Newton Michele  Watkins Guy R  Zou Ping  Germane Katherine L  McCorvey Lisa R  Wadzinski Brian E  Spiller Benjamin W
Institution:Department of Pharmacology, Vanderbilt University, Nashville, Tennessee 37232, USA.
Abstract:Protein phosphatase 2A (PP2A) is regulated through a variety of mechanisms, including post-translational modifications and association with regulatory proteins. Alpha4 is one such regulatory protein that binds the PP2A catalytic subunit (PP2Ac) and protects it from polyubiquitination and degradation. Alpha4 is a multidomain protein with a C-terminal domain that binds Mid1, a putative E3 ubiquitin ligase, and an N-terminal domain containing the PP2Ac-binding site. In this work, we present the structure of the N-terminal domain of mammalian Alpha4 determined by x-ray crystallography and use double electron-electron resonance spectroscopy to show that it is a flexible tetratricopeptide repeat-like protein. Structurally, Alpha4 differs from its yeast homolog, Tap42, in two important ways: 1) the position of the helix containing the PP2Ac-binding residues is in a more open conformation, showing flexibility in this region; and 2) Alpha4 contains a ubiquitin-interacting motif. The effects of wild-type and mutant Alpha4 on PP2Ac ubiquitination and stability were examined in mammalian cells by performing tandem ubiquitin-binding entity precipitations and cycloheximide chase experiments. Our results reveal that both the C-terminal Mid1-binding domain and the PP2Ac-binding determinants are required for Alpha4-mediated protection of PP2Ac from polyubiquitination and degradation.
Keywords:Adaptor Proteins  Phosphatase  Protein Degradation  Protein Phosphatase  Protein Structure  Protein Turnover  Ubiquitin  Ubiquitin Ligase
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