Apolipoprotein A-I, a hyperosmotic adaptation-related protein in ayu (Plecoglossus altivelis)

Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoprotein (HDL) particles in serum and participates in the reverse transport of cholesterol from tissues to the liver for excretion. 2DE gel and protein identification by MALDI-TOF-MS revealed that an apoA-I-like molecule was significantly decreased in the liver of ayu transferred from freshwater (FW) to brackish water (BW). A full-length cDNA clone of this protein was subsequently isolated. It contains 1209 bp with an open reading frame of 825 bp, coding for 275 amino acids with MW 31.1 kDa and pI 5.25. Ayu apoA-I had highest similarity (88.6% amino acid identity) to that of rainbow smelt. Phylogenetic analysis showed that vertebrate apoA-Is formed two major groups, representing mammalian–bird–amphibian (M–A) and fish apoA-Is respectively. Ayu apoA-I was most closely related to rainbow smelt apoA-I. In FW ayu, apoA-I transcripts were present in all tested tissues including brain, spleen, liver, kidney, gill, muscle, heart and intestine, and highest in liver, brain and intestine. In BW ayu, the expression levels of apoA-I gene were significantly decreased and almost totally inhibited in spleen, kidney, gill, heart and muscle. Significant down-regulation of proteins and mRNA in ayu transferred from FW to BW suggests an involvement in hyperosmotic regulation in this species.