Silicon transporter proteins

Diatoms were also investigated for the mechanism of silicon transport that is an integral part of the silicification process. As the environmental concentrations of dissolved silicon are rather low, diatoms must have an efficient transport system. Orthosilicic acid must not only be transported into the cell, but also transported intracellularly into the SDV where silica formation occurs. A protein of the diatom C. fusiformis was characterized that transports silicon from seawater into the cell. This discovery was accomplished by cloning and characterizing the DNA that codes for this protein. However, silicon transporter proteins of this particular type are not necessarily involved in intracellular transport. 

In Nature, specific silicon transport proteins (SITs) produced by diatoms are responsible for the uptake and delivery of orthosUicic acid (Si(OH)4, pIQ = 9.8) to the diatom. Within the cell, orthosihcic acid is then concentrated up to 1000-fold, resulting in the condensation of amorphous, hydrated sihca [45]. Strikingly, the... 

The Si transporting proteins (structures of which are deduced from the cloned DNA sequences) are closely related in structure to other, well-characterized ion transporters14. They contain 12 a-helical transmembrane domains that fold to form a cylindrical channel— assembled like staves of a barrel — through the lipid bilayer membrane of the cell. However, the evidence suggesting an ion-type transporter needs to be resolved with results of recent physiological analyses of the pH-dependence of silicon uptake, suggesting that many diatom species most efficiently take up the unionized, neutral silicic acid16. 

Because of the importance of diatoms in oceanic productivity, silicon is an important algal nutrient in seawater. A transporter of Si(OH)4 has been isolated and sequenced (Hildebrand et al, 1998 Hildebrand et al, 1997) and the physiology of silicon uptake has been well studied (Martin-Jezequel et al, 2000). Nonetheless, the molecular mechanism of Si(OH)4 transport and silica fmstule formation in diatoms are still largely mysterious. From indirect evidence, it appears possible that the Si(OH)4 transporter may contain zinc, coordinated to cysteines, as a metal center in the portion of the protein exposed to the outside of the cell (Hildebrand, 2000 Rueter and Morel, 1981). If true, this would be an unusual example of a transport protein functioning with a metal center. 

In contrast to C and N, Si-metabolism is metabolicaUy inexpensive for a given cell size, a silicon frustule requires ca. 1/10 the total metabolic energy of an equivalent cell wall composed of carbon compounds (Raven, 1983). Si-metabolism is also not directly coupled to photosynthesis. Si-transport and deposition are driven by oxidative phosphorylation (Blank and SuUivan, 1979 Blank et al, 1986 Sullivan, 1976), while serine and glycine, the amino acids which provide the main fraction of the protein matrix for Si-deposition (Werner, 1977), are terminal substances produced during photorespiration (Burris, 1977). Si-metaboHsm is instead Hnked to the regulation of cell division (Fig. 37.2) and growth rates (Brzezinski et al, 1990 Martin-Jezequel et al, 2000 Volcani, 1981). 

The use of silicone materials in contact lenses to improve oxygen transport through the lens to the cornea is well known in the field. It is also well known [43] that the dimethylsiloxanyl units selectively accumulate at hydrophobic surfaces during film formation. The surface properties of a lens can be quite dependent on the siloxane surface. Proteins are usually effectively repelled by such a surface. However, a siloxane surface is quite hydrophobic and, consequently, not very wettable. This often results in discomfort and lipid deposit formation with contact lenses. 

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