α2-Macroglobulin in the marine fish Sparus aurata

The α2-macroglobulin proteinase inhibitors (α2Ms) are a family of plasma proteins with the unique ability to inhibit a broad spectrum of proteinases, but are also known as binding proteins for many growth factors and cytokines, including growth hormone and members of the transforming growth factor-β superfamily. A partial cDNA (475 amino acids) encoding the C-terminus of α2M was cloned from the liver of the marine teleostean fish Sparus aurata. The deduced amino acid sequence of the cloned fragment showed 58-60% similarity to carp α2Ms. Northern blot analysis of hepatic α2M revealed a transcript of about 5 kb. A transcript of a similar size was detected in 1-day larvae. Steady state levels of α2M in larvae increased gradually on subsequent days post-hatching. α2M expression in embryos was determined by RT-PCR and started in embryos aged 8 h post-fertilization, but not earlier. RT-PCR of muscle RNA detected α2M also in fish muscle, albeit with a lower expression than in the liver. Immunoreactive-α2M was found in yolk syncytial layer of 3-day larvae and in livers from larvae and adults. Immunoreactive-α2M was also identified in soluble total proteins from young larvae with a pattern resembling that of plasma. These data demonstrate that the α2M gene is expressed early in fish development. Moreover, in addition to its major expression in liver, α2M is expressed also in fish muscle.