2.A.32 The Silicon Transporter (SIT) Family
Marine diatoms such as Cylindrotheca fusiformis encode at least six silicon transport protein homologues which exhibit similar sizes (about 550 aas) and topologies (twelve putative transmembrane α-helical spanners). The six homologues exhibit greater than 80% sequence identity with each other and therefore comprise a coherent family of closely related proteins. The six recognized SIT family members exhibit insufficient sequence similarity with other proteins in the databases to suggest homology. One characterized member of the family (Sit1) functions in the energy-dependent uptake of either Silicic acid [Si(OH)4] or Silicate [Si(OH)3O-] by a Na+ symport mechanism. The system is found in marine diatoms which make their 'glass houses' out of silicon. Homologues of Sit1 are expressed at different levels and in different patterns. Multiple Sit1 homologues are found in all diatoms tested. Individual transporters probably play specific roles in silicon uptake. Thamatrakoln and Hildebrand (2005) have reviewed the available evidence concerning SIT family members. Members of the SIT family have been identified in plants, siliceous sponges and choanoflagellates (Marron et al. 2016).
The generalized transport reaction is:
Silicate (out) + nNa+ (out) → Silicate (in) + nNa+ (in)