Polyhydroxyalkanoate (PHA) Granules Have no Phospholipids.

Area of Science:

• Microbiology
• Biochemistry
• Cell Biology

Background:

• Polyhydroxybutyrate (PHB) granules, termed carbonosomes, are prokaryotic inclusions with a PHB polymer core and a protein surface layer.
• In vitro studies suggest phospholipids are present in the PHB granule surface, but in vivo evidence is lacking.

Purpose of the Study:

• To investigate the in vivo presence of phospholipids on polyhydroxyalkanoate (PHA) granule surfaces in prokaryotes.
• To determine if the PHB granule surface layer is composed solely of proteins.

Main Methods:

• Expression of fluorescent protein fusions with the phospholipid-binding LactC2 domain in model organisms (Ralstonia eutropha, Pseudomonas putida, Magnetospirillum gryphiswaldense).
• Localization studies using fluorescence microscopy to assess co-localization with PHB/PHA granules and cellular membranes.

Main Results:

• Fusion proteins localized to the cell membrane but not to PHB granules in R. eutropha and P. putida.
• In M. gryphiswaldense, fusion proteins localized to the cytoplasmic membrane and magnetosome chains, but not PHB granules.
• Demonstrated absence of phospholipids in vivo on PHB/PHA granules across different bacterial classes.

Conclusions:

• The in vivo carbonosomes of α-, β-, and γ-proteobacteria lack a phospholipid layer.
• Postulates that PHB/PHA granule surfaces in natural producers are generally protein-only, refuting the presence of phospholipids.