Polyhydroxyalkanoates (PHAs) are natural polyesters synthesized by numerous microorganisms as energy and reducing power storage materials, and have attracted much attention as substitutes for petroleum-based plastics. Here, we report the first crystal structure of Ralstonia eutropha PHA synthase at 1.8 Å resolution and structure-based mechanisms for PHA polymerization. RePhaC1 contains two distinct domains, the N-terminal (RePhaC1 ) and C-terminal domains (RePhaC1 ), and exists as a dimer. RePhaC1 catalyzes polymerization via non-processive ping-pong mechanism using a Cys-His-Asp catalytic triad. Molecular docking simulation of 3-hydroxybutyryl-CoA to the active site of RePhaC1 reveals residues involved in the formation of 3-hydroxybutyryl-CoA binding pocket and substrate binding tunnel. Comparative analysis with other polymerases elucidates how different classes of PHA synthases show different substrate specificities. Furthermore, we attempted structure-based protein engineering and developed a RePhaC1 mutant with enhanced PHA synthase activity.

Polyhydroxyalkanoates (PHAs) are natural polyesters synthesized by numerous microorganisms as energy and reducing power storage materials, and have attracted much attention as substitutes for petroleum-based plastics. In an accompanying paper, the authors reported the crystal structure of the C-terminal domain of Ralstonia eutropha PHA synthase (PhaC1). Here, the authors report the 3D reconstructed model of full-length of R. eutropha PhaC1 (RePhaC1 ) by small angle X-ray scattering (SAXS) analysis. The catalytic C-terminal domain of RePhaC1 (RePhaC1 ) dimer is located at the center of RePhaC1 , and the N-terminal domain of RePhaC1 (RePhaC1 ) is located opposite the dimerization subdomain of RePhaC1 , indicating that RePhaC1 is not directly involved in the enzyme catalysis. The localization studies using RePhaC1 , RePhaC1 and RePhaC1 revealed that RePhaC1 plays important roles in PHA polymerization by localizing the enzyme to the PHA granules and stabilizing the growing PHA polymer near the active site of RePhaC1 . The serial truncation study on RePhaC1 suggested that the predicted five α-helices (N-α3 to N-α7) are required for proper folding and granule binding function of RePhaC1 . In addition, the authors also report the SAXS 3D reconstructed model of the RePhaC1 /RePhaM complex (RePhaM , PAKKA motif-truncated version of RePhaM). RePhaM forms a complex with RePhaC1 by interacting with RePhaC1 and activates RePhaC1 by providing a more extensive surface area for interaction with the growing PHA polymer.

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The hPrp19-CDC5L complex plays a crucial role during human pre-mRNA splicing by catalytic activation of the spliceosome. In order to elucidate the molecular architecture of the hPrp19-CDC5L complex, the crystal structure of CTNNBL1, one of the major components of this complex, was determined. Unlike canonical ARM-repeat proteins such as β-catenin and importin-α, CTNNBL1 was found to contain a twisted and extended ARM-repeat structure at the C-terminal domain and, more importantly, the protein formed a stable dimer. A highly negatively charged patch formed in the N-terminal ARM-repeat domain of CTNNBL1 provides a binding site for CDC5L, a binding partner of the protein in the hPrp19-CDC5L complex, and these two proteins form a complex with a stoichiometry of 2:2. These findings not only present the crystal structure of a novel ARM-repeat protein, CTNNBL1, but also provide insights into the detailed molecular architecture of the hPrp19-CDC5L complex.

Polyhydroxyalkanoates (PHAs) are a family of polyesters synthesized and intracellularly accumulated as distinct granules in many microorganisms. PHAs are promising materials to substitute the petroleum‐based plastics currently in use. In article 1907138, Sang Yup Lee and co‐workers provide a comprehensive overview of PHAs, including their biosynthesis, overall production process, and applications.