Structure/Function Studies of Human Striatin

Protein phosphatase 2A (PP2A) is involved in the regulation of cellular proliferation and apoptosis. It is a heterotrimer composed of a structural A subunit, a catalytic C subunit, and a regulatory B‐type subunit. The B‐type subunits direct PP2A to specific substrates in different signal transduction pathways. The known classes of B‐type subunits are designated B, B′, and B″. Striatin, a 110 kD protein originally purified from rat brain tissue, has been identified as a putative B‴ subunit. Striatin contains several protein‐binding domains, indicating it serves as a scaffolding protein in multiprotein complexes. Striatin binds PP2A and proteins that may be PP2A substrates, a number of which are involved in signaling cascades. Striatin has also been implicated in protein trafficking. For example, striatin binding to ERα results in translocation of nuclear ERα to the plasma membrane. Mutational studies of striatin were performed to learn what elements are required for protein‐protein interactions and how these interactions affect PP2A. Co‐immunoprecipitation and immunoblotting experiments comparing wild‐type striatin to our mutants has yielded important data regarding the requirements for interactions between striatin and some of its targets. We have found that PP2A binding resides within the N‐terminus of striatin. mMOB1, or phocein, is known to bind to striatin‐PP2A complexes. All of our mutants retained phocein binding, suggesting the presence of at least two binding sites on striatin. HEK293 cells were transiently transfected with plasmids expressing GFP‐striatin fusion proteins to determine the molecular requirements for correct subcellular localization. The GFP‐fusions were excluded from the nucleus, except for mutants lacking residues 270–309, suggesting the presence of a nuclear export signal or a protein‐binding site within this region.