An inhibitor κB homolog from the bivalve mollusc solen grandis that responds to immune challenge

The nuclear factor-κB (NF-κB) signaling pathway has been studied extensively in mammalians and insects but has been less well investigated in marine molluscs. Inhibitor of κB (IκB), an important component of the NF-κB signaling pathway, serves as a crucial mediator of the innate immune system. A homolog of IκB was identified in a razor clam (Solen grandis), designated as SgIκB, and its messenger RNA expression was detected both in tissues and towards pathogen-associated molecular patterns. Full-length complementaryDNAof SgIκB is 2,232 bp, containing a 181-bp 5' untranslated region (UTR) and a 970-bp 3# UTR with a poly (A) tail. The open reading frame is 1,080 bp, encoding a 359-amino acid polypeptide with a predicted molecular weight of 40.1 kDa and an isoelectric point of 4.88. A potential PEST motif (E₂SNDLEMDTCPLEMDS17) and the IκB degradation motif (ES₄₄GYKS₄₈) are located at the N-terminus, and 2 conserved casein kinase II phosphorylation sites (S₃₃₇DEE3₄₀ and S₃₄₆YDD₃₄₉) exist at the C terminus. The presence of 6 conserved ankyrin repeats in SgIκB and its close phylogenetic relationship with other IκBs strongly suggest that SgIκB belongs to the IκB superfamily. Messenger RNA of SgIκB is expressed constitutively in various tissues of healthy S. grandis, with the greatest expression in gill and hepatopancreas, followed by gonad, mantle, hemocyte, and muscle in descending order. Messenger RNA expression of SgIκB in hemocytes is upregulated significantly to varying degrees (P < 0.01) on stimulation with lipopolysaccharide, peptidoglycan, and β-1,3-glucan. The results indicate the existence of a NF-κB signaling pathway in S. grandis and provide evidence for possible regulatory mechanisms during an immune challenge.