The outstanding feature of a diatom is the species-specific design and ornamentation of the silica-based cell wall, termed frustulum. A new frustulum is shaped in a specialized organelle (silica deposition vesicle) and secreted. Proteins in the lumen of this organelle may control the biomineralization process and are likely to remain associated with the mature cell wall. Therefore a study of the structures of proteins associated with the diatom cell wall was initiated. The complete primary structures of three cell wall proteins (denoted as frustulins) have been determined. In addition, partial amino acid sequences from two more cell wall components were obtained. From these data, a highly conserved domain has been identified as a common building block of diatom cell wall proteins that is repeated several time; per polypeptide chain together with polyproline/hydroxyproline or polyglycine spacers. All frustulins characterized so far, are synthesized as preproteins with a novel type of N-terminal presequence.