Molecular response to TBT stress in marine sponge Suberites domuncula: proteolytical cleavage and phosphorylation of KRS_SD protein kinase

Abstract

Marine sponges as sessile filter feeders are inevitably under a constant influence of changes in their environment. Mediation of extracellular signals and regulation of cellular response to environmental stress is a key function of cellular protein kinases. Expression, proteolytical cleavage and phosphorylation of stress-responsive KRS_SD protein kinase, in control and tributyl-tin (TBT) treated sponges were investigated. In control sponge, two KRS_SD proteins were expressed: KRS_SD1 (54 kDa) corresponding to KRS_SD calculated molecular weight, and KRS_SD2 (50 kDa). Exposure of sponges to TBT resulted in alteration of KRS_SD1 and KRS_SD2 expression levels and their phosphorylation state. KRS_SD1 band disappearance after 4 h coincides with the appearance of additional 35 kDa immunoreactive protein band (p35) and decrease in number of viable cells. Incubation of recombinant KRS_SD with protein extracts from TBT stressed sponges resulted in cleavage of recombinant KRS_SD protein. The size of the resulting protein fragment suggests caspase-3 recognition site, present in both recombinant KRS_SD and KRS_SD, as a possible site of cleavage. Phosphorylated KRS_SD bands appear at the same time as p35 and remain after p35 band intensity reached control level. KRS_SD induction, its phosphorylation and proteolytical cleavage during TBT stress suggest that in sponge cells exist mechanism similar to one present in human cells where KRS/MST protein kinase is involved in promotion of apoptosis following oxidative stress.