Identification and characterization of diatom kinases catalyzing the phosphorylation of biomineral forming proteins

Abstract

Diatoms are unicellular photosynthetic algae that display intricately patterned cell walls made of amorphous silicon dioxide (silica). Long-chain polyamines and highly phosphorylated proteins, silaffins and silacidins, are believed to play an important role in biosilica formation. The phosphate moieties on silaffins and silacidins play a significant role in biomineral formation, yet no kinase has been identified that phosphorylates these biomineral forming proteins. This dissertation describes the characterization of a novel kinase from the diatom Thalassiosira pseudonana, tpSTK1, which is upregulated during silica formation. A recombinantly expressed histidine-tagged version of tpSTK1 was capable of phosphorylating recombinant silaffins but not recombinant silacidin in vitro. Through establishing methods for subcellular fraction of T. pseudonana membranes in combination with antibody inhibition assay, it was discovered that native tpSTK1 phosphorylates silaffins but not silacidins in vitro (i.e. it exhibits the same substrate specificity as recombinant tpSTK1). As tpSTK1 is an abundant protein in the ER lumen (~ 0.5 % of total ER protein) it seems highly likely to function as a silaffin kinase in vivo. TpSTK1 lacks clear sequence homologs in non-diatom organisms and is the first molecularly characterized kinase that appears to be involved in biomineralization. The predicted kinase domain (KD) of tpSTK2, the only T. pseudonana homolog of tpSTK1, was recombinantly expressed and tested for phosphorylation activity. Recombinant tpSTK2-KD and native tpSTK2 exhibited detectable activity with myelin basic protein, but did not phosphorylate silaffins or silacidins in vitro. Western blot analysis demonstrated that native tpSTK2 was not present in the ER, but associated with the cytosol and Golgi membrane containing subcellular fractions.