Conserved motif CMLD in silicic acid transport proteins of diatoms

Sequenced fragments of genes coding for silicon transporters (SITs) were analyzed for diatoms of evolutionarily distant classes (centric Chaetoceros muelleri Lemmermann, pennate araphid Synedra acus Kutzing, pennate raphid Phaeodactylum tricornutum Bohlin, and pennate Cylindrotheca fusiformis Reimann et Lewin with a keeled raphe system). SITs were found to contain a conserved motif, CMLD. Hydropathy profiles showed that the motif CMLD is between two transmembrane domains lacking Lys and Arg, and the domains were consequently assumed to play a role in the formation of a channel mediating silicic acid transport. The motif CMLD proved to be rare. Since Zn2+ is necessary for silica incorporation into diatom cells, a hypothesis was advanced that the motif CMLD acts as a Zn-binding site. Diatom growth suppression was observed in the presence of the alkylating agent N-iodoacetylamidoethyl-1-aminonaphthalene-5-sulfonic acid (AEDANS), which does not penetrate into the cell. Cys of the motif CMLD was assumed to act as a target for AEDANS. Zinc ions inhibited Cys alkylation in the synthetic peptide NCMLDY, testifying to the above hypothesis.