Novel Ser74 of NF-kappaB/CgIkappaBalpha Phosphorylated by MAPK/ERK Regulates Temperature Adaptation in Oysters

Phosphorylation of I{kappa}B at Ser32 and Ser36 by IKKs during biotic stress triggers its ubiquitin-proteasome degradation, causing to the nuclear translocation of REL, representing a key cascade mechanism in metazoans conserved and immune core signaling pathway, NF-{kappa}B. However, studies on its response to abiotic stress and signal transduction by phosphorylation in mollusks are lacking. Here, we firstly report a novel heat-induced phosphorylation site (Ser74) at the major NF-{kappa}B/CgI{kappa}B of oysters, phosphorylated by MAPK/CgERK1/2, which independently mediated the subsequent ubiquitin-proteasome degradation without phosphorylation at Ser32 and Ser36 and decreased thermal stability. The degradation of CgI{kappa}B promoted CgREL nuclear translocation, which stimulated cell survival related gene expression to defend against thermal stress. The MAPK and NF-{kappa}B pathways exhibited stronger activation patterns in higher environmental temperature and in the warm-adapted Crassostrea angulata than those in the cold-adapted C. gigas-two allopatric congeneric oyster species with differential habitat temperatures. These findings unveil the complex and unique phosphorylation-mediated signal transduction mechanisms in marine invertebrates, and further expand our understanding of the evolution and function of established classical pathway crosstalk mechanisms.