Polyester synthases and polyester granule assembly : a thesis presented to Massey University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Microbiology
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Date
2008
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Massey University
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Abstract
PHAs are a class of biopolymers consisting of (R)-3-hydroxy-fatty acids and are produced by the
majority of eubacteria and some archaeal bacteria as carbon storage material. In general, PHA is
synthesised when a carbon source is available in excess while another essential nutrient is limited.
The key enzyme of PHA biosynthesis, the PHA synthase, catalyses the polymerisation of the
substrate (R)-3-hydroxyacyl-CoA to PHA accompanied by the release of coenzyme A. PHA is
stored intracellularly as inclusions, the so-called PHA granules. When the external carbon source
becomes exhausted, bacteria can metabolise these carbon inclusions by degradation of the
polymer.
PHA granules are water-insoluble, spherical inclusions of approximately 50-500 nm in diameter
which consist of a hydrophobic polyester core surrounded by a phospholipid layer with
embedded and attached proteins. One could consider isolated PHA granules as bio-beads due to
their structure and size. In this study we tested if the PHA synthase can be used as an anchor
molecule in order to display proteins of interest at the PHA granule surface. Furthermore, these
modified PHA granules were analysed for their potential applicability as bio-beads in
biotechnological procedures. The concept of using the PHA synthase as granule-anchoring
molecule for display of proteins of interest was established by the functional display of the ß-
galactosidase at PHA granules. This “proof of concept” was followed by the display of
biotechnologically more interesting proteins. The IgG binding domain of protein A as well as
streptavidin, which is known for its biotin binding ability, were fused to the PHA synthase,
respectively, and therefore localised at the PHA granule surfaces during PHA granule assembly,
resulting in functional bio-protein A -beads and bio-streptavidin-beads. Moreover, their
applicability in biotechnological assays was demonstrated.
Recently, we fused the green fluorescent protein (GFP) to the PHA synthase and demonstrated
that the PHA granule assembly does not start randomly distributed in the cytoplasm but occurred
localised at or near the cell poles. To further investigate if the localisation of the PHA granule
formation process is due to polar positional information inherent to the PHA synthase, different
mutated versions of the PHA synthase of Cupriavidus necator were created and analysed for a
potential alteration in localisation. Furthermore, the phasin protein PhaP1 of C. necator was fused
to HcRed, a far-red fluorescent protein, and localisation studies were accomplished when the
fusion protein was expressed under different conditions in Escherichia coli.
Description
Keywords
PHA biosynthesis, PHA granule assembly, Microbial polymers, Polyhydroxyalkanoate synthase, Protein engineering, Bio-beads