Identification of a small HSP gene from hard clam Meretrix meretrix and its potential as an environmental stress biomarker

Heat shock proteins (HSPs) are molecular chaperones that help organisms cope with stressful conditions. In this study, a novel small HSP (sHSP) gene was identified from the hard clam Meretrix meretrix (designated as Mm-HSP20). The full length of the cDNA is 1222 bp, con- sisting of a 5'-terminal untranslated region (5'UTR) of 103 bp, a 3'UTR of 565 bp, and a 522 bp open reading frame encoding a polypeptide of 173 amino acids. Sequence comparison showed that Mm-HSP20 had a moderate degree of homology to the sHSP of other organisms. A sHSP feature domain, an alpha-crystallin domain, and a V/IXI/V motif in the C-terminal extension were identified in the Mm-HSP20 amino acid sequence, indicating that Mm-HSP20 is a new member of the sHSP family. The Mm-HSP20 transcript was constitutively expressed in 6 different test tissues, with the highest expression level detected in the digestive gland by fluorescent quantitative real- time PCR. The temporal expression of Mm-HSP20 mRNA in the digestive gland and hemocytes was evaluated after exposure to cadmium (Cd) (40 µg l �1 CdCl2) and benzo(a)pyrene (50 µg l �1 B(a)P) individually, and in combination (40 µg l �1 CdCl2 +5 0 µg l �1 B(a)P). Mm-HSP20 expression level increased significantly after Cd and B(a)P exposure in both tissue types. Conversely, signifi- cant Mm-HSP20 transcript repression was detected in the 24 h B(a)P-treated digestive gland sam- ples. Considerable up-regulation of Mm-HSP20 mRNA level was observed after multiple expo- sures. These results indicate that Mm-HSP20 plays a role in mechanisms involved in coping with environmental stress in hard clams. Mm-HSP20 may be suitable for use as a biomarker for heavy metal and B(a)P contamination.