Lessons from Biomineralisation: the Role of Post-translational Modification
Authors:   , Matthewson Benjamin 
Published: November 2021

<p>In this thesis we present our findings following analysis of the acidic organic matrix (SMP) occluded in the calcite spines of the New Zealand sea urchin Evechinus chloroticus. The main focus involves correlation of the structure and function of the post-translational modifications (PTMs). The experimental framework developed to achieve this involved mapping the structure of the PTMs throughout SMP based on molecular weight (MW) followed by selective removal of each of the identified PTMs. The functional analysis involved the use of SMP, and its derivatives, as additives in an in vitro calcium carbonate crystallisation assay. The adoption of in vitro methods was considered appropriate as the focus of this work was to develop strategies towards programmable crystal growth in vitro. From analysis of the PTMs we have shown that there is extensive protein glycosylation, sulfation, and phosphorylation; all are involved in rendering the isoelectric point (pI) of the SMP macromolecules. The sulfates are exclusively housed on the glycan framework whereas the phosphate is protein bound. The majority of the SMP glycone is charged with O-glycosylation accounting for 80.0 +/- 4.0 wt%. The structure of the glycans includes sulfated HexNAc oligomers, and potentially mucin-like/keratan sulfate and/or carrageenan structures. Using Stains-All we have shown that the desulfated HexNAc oligomers have the ability to bind calcium which signals relevance in the formation of calcium carbonate. SMP was fractionated by MW across a series of spin-filters. Use of the various fractions in the crystallisation assay showed that the species in the greater than 30 kDa fraction held the ability to increase the number of crystals nucleated. In contrast, the macromolecules in the 10 to 30 kDa range contained the full complement of morphologically active species. The result that these functions can be isolated demonstrates that they are independently controlled. The structure-function relationships determined include: the protein and the acidic glycans are jointly sufficient to generate the nucleating function; deglycosylated SMP holds the complete morphological activity, however, the glycans contribute by increasing reproducibility presumably through regulatory influences; and the sterically hindered phosphate residues make a slight contribution to this morphological activity. These results indicate that analyses which involve characterisation of the morphological function of cloned biomineral proteins may indeed correspond to their native counterparts. The observation that the morphologically active species are phosphorylated identifies them as the calcium-binding phosphoproteins. The morphological activity of SMP stripped of all PTMs is equivalent to the proteins extracted from the aragonitic layer of Haliotis iris. Characterisation of SMP demonstrated similarities with the OMs of other sea urchin species. For example, SMP appears to include SM30. In addition, the overall structure of SMP includes abundant acidic glycosylation with a relatively neutral protein component. This structural make-up is in contrast to the highly acidic proteins which are barely post-translationally modified.</p>