Two homologues of inhibitor of NF-kappa B (IκB) are involved in the immune defense of the Pacific oyster, Crassostrea gigas.

A novel homologue of IκB was cloned from a hemocyte cDNA of Crassostrea gigas (designed as CgIκB2). The complete cDNA of CgIκB2 includes an open reading frame (ORF) of 1032 bp, and 3' and 5'untranslated regions (UTR's) of 141 bp and 279 bp, respectively. The ORF encodes a putative protein of 343 amino acids with a calculated molecular weight of approximately 37.8 kDa. Alignment analysis reveals that CgIκB2 contains a conserved degradation motif and six ankyrin repeats. A phylogenetic analysis suggests that a gene duplication event prior to the gastropod-bivalve divergence resulted in the emergence of two IκB homologues in C. gigas. Distinct maximal expression patterns of CgIκB1 in hemocytes and CgIκB2 in the gonad were observed. CgIκB1 and CgIκB2 expression in response to bacterial challenge is similar and inducible. Moreover, both CgIκB1 and CgIκB2 are able to inhibit NF-κb/Rel activating transcription in S2 or HEK293 cells. Our findings demonstrate that both CgIκB1 and CgIκB2 are involved in immune defense in C. gigas through regulation of NF-κB/Rel activity.