Fig 1.

Hypothetical pathway for intracellular transport of silaffin-enzyme fusion proteins to the silica deposition vesicle (SDV) (23, 36). The fusion proteins are cotranslationally imported into the endoplasmic reticulum (ER), and the signal peptide (SP) for ER import is removed by signal peptidase. Further modifications of the fusion proteins may occur in the ER and after transport to and through the Golgi apparatus. The fusion proteins may reach the SDV via specific transport vesicles, and become incorporated into the silica-forming organic matrix in the SDV lumen. After completion of silica formation, the silaffin-enzyme fusion proteins become trapped in the deposited silica and, after exocytosis of the SDV contents, remain stably attached to the biosilica cell wall.