Metallothionein CgMTIII is involved in zinc binding and accumulation in the Pacific oyster Crassostrea gigas

Abstract

Metallothioneins (MTs) are a family of small, metal-binding proteins that play a pivotal role in metal storage and detoxification. The Pacific oyster Crassostrea gigas is known for its exceptionally high zinc content, and investigating the role of MTs in oyster zinc metabolism will help elucidate the mechanisms underlying its zinc accumulation. In this study, a MT homolog CgMTIII was cloned from C. gigas to investigate its function in zinc binding and accumulation. The transcripts of CgMTIII were distributed in all examined oyster tissues including gill, mantle, adductor muscle, hepatopancreas, gonad, labial palp and haemocytes, with the highest expression in the gill and the lowest in the haemocytes. Immobilized metal ion affinity chromatography (IMAC) demonstrated that the recombinant CgMTIII protein exhibited high retention on the zinc-charged column, confirming its zinc-binding activity. Overexpression of CgMTIII in HEK293T cells led to a 2.34-fold increase in intracellular zinc content after 24 h of exposure to 100 μM zinc, compared to the control group. Moreover, knockdown of CgMTIII through RNA interference resulted in a 24% reduction in zinc content in the gill tissue of oysters. Collectively, CgMTIII exhibited zinc-binding activity and contributed significantly to zinc accumulation in C. gigas. These findings deepen our understanding of the zinc-enrichment mechanism in oysters and provide a theoretical foundation for breeding high‑zinc oyster varieties.