Possible role of ubiquitin in silica biomineralization in diatoms: identification of a homologue with high silica affinity

In diatom silicon biomineralization peptides are believed to play a role in silica precipitation and the consequent structure direction of the cell wall. Characterization of such peptides should reveal the nature of this organic-inorganic interaction, knowledge that may eventually well be used to expand the existing range of artificial silicas ("biomimicking"). Biochemical studies on Navicula pelliculosa revealed a set of proteins, which have a high affinity for a solid silica matrix; some were only eluted from the matrix when SDS-denaturation was applied. One of the proteins with an affinity for silica, about 8.5 kDa, is shown to be a homologue of ubiquitin on the basis of its N-terminal amino acid sequence; ubiquitin itself is a highly conserved 8.6 kDa protein that is involved in protein degradation. This finding is in line with a model of silica biomineralization in diatoms that implies the removal of templating polypeptides when pores in the growing cell wall develop. Western blotting with specific anti-ubiquitin antibodies confirmed cross-reactivity. Immunocytochemical localization of ubiquitin indicates that it is present along the diatom cell wall and inside pores during different stages of valve formation.