Hypoxia caused by global climate change and anthropogenic pollution has exposed marine species to increasing stress.
Oxygen sensing mediated by
prolyl hydroxylase (PHD) is regarded as the first line of defense under
hypoxia exposure; however, the function of PHD in marine molluscan species remains unclear. In this study, we identified two PHD2 gene in the oyster Crassostrea gigas using phylogenetic tree analysis with 36 species, namely, CgPHD2A/B. Under
hypoxia, the
mRNA and
protein expression of CgPHD2A displayed a time-dependent pattern, revealing a critical role in the response to
hypoxia-induced stress. Observation of interactions between CgPHD2 and CgHIF-1α
proteins under normoxia using co-immunoprecipitation and GST-pull down experiments showed that the β2β3 loop in CgPHD2A hydroxylates CgHIF-1α to promote its ubiquitination with CgVHL. With the
protein recombination and site-directed mutagenesis, the hydroxylation domain and two target
proline loci (P404A and 504A) in CgPHDs and CgHIF-1α were identified respectively. Moreover, the electrophoretic mobility-shift assay (EMSA) and
luciferase double reporter gene assay revelaed that CgHIF-1α could regulate CgPHD2A expression through binding with the
hypoxia-responsive
element in the promoter region (320 bp upstream), forming a feedback loop. However,
protein structure analysis indicated that six extra
amino acids formed an α-helix in the β2β3 loop of CgPHD2B, inhibiting its activity. Overall, this study revealed that two CgPHD2
proteins have evolved, which encode
enzymes with different activities in oyster, potentially representing a specific
hypoxia-sensing mechanism in mollusks. Illustrating the functional diversity of CgPHDs could help to assess the physiological status of oyster and guide their aquaculture.