Patric Turowski

The Structure of the Protein Phosphatase 2A PR65/A Subunit Reveals the Conformation of Its 15 Tandemly Repeated HEAT Motifs

The PR65/A subunit of protein phosphatase 2A serves as a scaffolding molecule to coordinate the assembly of the catalytic subunit and a variable regulatory B subunit, generating functionally diverse heterotrimers. Mutations of the beta isoform of PR65 are associated with lung and colon tumors. The crystal structure of the PR65/Aalpha subunit, at 2.3 A resolution, reveals the conformation of its 15 tandemly repeated HEAT sequences, degenerate motifs of approximately 39 amino acids present in a variety of proteins, including huntingtin and importin beta. Individual motifs are composed of a pair of antiparallel alpha helices that assemble in a mainly linear, repetitive fashion to form an elongated molecule characterized by a double layer of alpha helices. Left-handed rotations at three interrepeat interfaces generate a novel left-hand superhelical conformation. The protein interaction interface is formed from the intrarepeat turns that are aligned to form a continuous ridge.

Deregulation of translational control of the 65-kDa regulatory subunit (PR65 alpha) of protein phosphatase 2A leads to multinucleated cells.

Efficient translation of the mRNA encoding the 65-kDa regulatory subunit (PR65α) of protein phosphatase 2A (PP2A) is prevented by an out of frame upstream AUG and a stable stem-loop structure (ΔG = −55.9 kcal/mol) in the 5′-untranslated region (5′-UTR). Deletion of the 5′-UTR allows efficient translation of the PR65α message in vitro and overexpression in COS-1 cells. Insertion of the 5′-UTR into the β-galactosidase leader sequence dramatically inhibits translation of the β-galactosidase message in vitro and in vivo, confirming that this sequence functions as a potent translation regulatory sequence. Cells transfected or microinjected with a PR65α expression vector lacking the 5′-UTR, express high levels of PR65α, accumulating in both nucleus and cytoplasm. PR65α overexpressing rat embryo fibroblasts (REF-52 cells) become multinucleated. These data and previous results (Mayer-Jaekel, R. E., Ohkura, H., Gomes, R., Sunkel, C. E., Baumgartner, S., Hemmings, B. A., and Glover, D. M.(1993) Cell 72, 621-633) suggest that PP2A participates in the regulation of both mitosis and cytokinesis.