The immunoglobulin genes in an Actinopterygian fish, the Siberian sturgeon (Acipenser baeri)

Sammanfattning: Characterization of the immunoglobulin (Ig) light (L) and heavy (H) chain genes from the Siberiansturgeon, Acipenser baeri, revealed an Ig structure simhar to the IgM elass of mammals. The serumforms of the sturgeon Ig are tetrameric and dimeric.The sturgeon IgL gene organization is also of the translocon type, with several variable (V)segments followed by some joining (J) segments and the constant (C) exon indicate that theformation of IgL multicluster organisations in teleosts and elasmobranchs are two separateevolutionary events. The sturgeon VL segments all belong to a single family with some 20 members.Light chain variability is generated by random rearrangement of the VL and J segments. The sturgeon IgL is classified as κ like based on homologies of the VL sequence, and similarities of the recombination signal sequence.The genomic organization of the IgH locus in sturgeon is of the translocon type. withmultiple V gene segments preceeding several D segments, a number of J segments followed by theCµ region. Two allelic variants of the µ chain have been identitied. The three different V families characterized display a high variability in the complementarity determining regions (CDR) 1 and 2, and participate in the generation of variability of CDR3 by random rerrangement to the D and J segments. The variability is increased by imprecise joining, the addition of P and N nucleotides, and exonuclease activity, which results in the use of the D segments in all reading frames.Two forms of alternative splicing to generate the membrane bound Ig was found in thesturgeon. The first is splicing the TMl exon to a cryptic splice site in the 3' part of the Cµ4 exon, which is common to all gnatostomes except the teleosts. The second is the result from competetive splicing between the Cµ42 exon and the TMl exon to the regular Cµ41 donor site, and is unique to the sturgeon.