New insights in the formation of polyhydroxyalkanoate granules (carbonosomes) and novel functions of poly(3‐hydroxybutyrate)

Abstract

SummaryThe metabolism of polyhydroxybutyrate (PHB) and related polyhydroxyalkanoates (PHAs) has been investigated by many groups for about three decades, and good progress was obtained in understanding the mechanisms of biosynthesis and biodegradation of this class of storage molecules. However, the molecular events that happen at the onset of PHB synthesis and the details of the initiation of PHB/PHA granule formation, as well as the complex composition of the proteinaceous surface layer of PHB/PHA granules, have only recently come into the focus of research and were not reviewed yet. In this contribution, we summarize the progress in understanding the initiation and formation of the PHA granule complex at the example of Ralstonia eutropha H16 (model organism of PHB-accumulating bacteria). Where appropriate, we include information on PHA granules of Pseudomonas putida as a representative species for medium-chain-length PHA-accumulating bacteria. We suggest to replace the previous micelle mode of PHB granule formation by the Scaffold Model in which the PHB synthase initiation complex is bound to the bacterial nucleoid. In the second part, we highlight data on other forms of PHB: oligo-PHB with ≈100 to 200 3-hydroxybutyrate (3HB) units and covalently bound PHB (cPHB) are unrelated in function to storage PHB but are presumably present in all living organisms, and therefore must be of fundamental importance.

Comparative Subcellular Localization Analysis of Magnetosome Proteins Reveals a Unique Localization Behavior of Mms6 Protein onto Magnetite Crystals

“…In various prokaryotes, intracellular compartments can be created to provide the domains required for highly specialized reactions. Whereas some of these compartments are completely proteinaceous (e.g., carboxysomes, metabolosomes, and ferritin) (1-3), others contain molecular components similar to those in cell membranes, including lipids and proteins (e.g., nucleoids, polyhydroxybutyrate, and spores) (4,5). Such compartmentalized organelles are recognized to be formed within bacteria through multiple processes involving the spatial regulation of protein localization, but the details of this regulatory machinery remain largely unknown (6).…”

Biodegradable Polymers: Recent Developments and New Perspectives

“…PHB is categorized into three types in accordance with the number of 3HB units and biological function: (i) high molecular weight PHB (1,000 to > 1,000,000 3HB units, storage PHB), (ii) low molecular weight PHB (≈100-200 3HB units, oligo-PHB), and (iii) conjugated PHB (< 30 3HB units, cPHB) [6][7][8]. Storage PHB is found in many prokaryotes, such as Eubacteria and Archaea, while oligo-PHB and cPHB are found in both prokaryotes and eukaryotes, that is, oligo-PHB and cPHB may be present in all organisms.…”

A Novel DNA-Binding Protein, PhaR, Plays a Central Role in the Regulation of Polyhydroxyalkanoate Accumulation and Granule Formation in the Haloarchaeon Haloferax mediterranei

“…Native PHA granules are found to be composed of 97.5% PHA, 2% proteins, and likely some amount of lipids (7). At least four types of proteins were found to be the PHA granule-associated proteins (PGAPs) in bacteria: PHA synthases, PHA depolymerases, regulators, and structural proteins (phasins [PhaPs]) (8,9). In recent years, increasing new roles have been found for the PGAPs.…”

“…The PGAPs play important roles in PHA synthesis, PHA utilization, and granule formation and distribution (8,9,12), among which the regulatory proteins are responsible for ensuring the proper formation of PHA granules by influencing the expression of both phasins and themselves (13)(14)(15)(16)(17). A classic regulation model was presented in a poly(3-hydroxybutyrate) (PHB [a type of PHA])-accumulating bacterium, Ralstonia eutropha H16 (9).…”