The organization of these gVLR genes differs depending on the gene and species (2b). The possible combinations of VLRA and VLRB are estimated to generate a potential repertoire that is almost equivalent to the TCRs

and BCRs of jawed vertebrates, (> 1014) [22]. This observation suggests that VLRs are the antigen receptors of jawless vertebrates. Consistent with this, lampreys immunized with human erythrocytes or anthrax spores of Bacillus anthracis produce antigen-specific soluble VLRB molecules that act as antibodies [22], [23]. These observations indicate that, despite their lack of structural similarity to the HSP inhibitor review antigen receptors of jawed vertebrates, VLRs function as antigen receptors in jawless vertebrates. During development of LLCs, LRR modules are inserted into the gVLR gene by a gene conversion-like

mechanism (2c) [19], [24]. Multiple LRRNT-, LRR1-, LRRV-, LRRVe-, CP- and LRRCT-encoding modules are located proximally to the gVLR gene. A homologous sequence is used to prime the insertion of those modules during VLR assembly. The sequences located at the ends of the most newly copied LRR module determine the next LRR module. In this way, LRR modules are unidirectionally inserted into the gVLR gene. Although, monoallelic assembly of the VLRA and VLRB genes occurs in the majority of cases, diallelic assembly has MG 132 been observed in a few cases [25]. In such instances, one mature VLR gene encodes a functional VLR structure, while the other does not. The

unsuccessful mature VLR gene contains an in-frame stop codon. These observations indicate that an inhibitory feedback mechanism regulates VLR assembly. The molecular mechanism of VLR assembly is still unknown. However, two CDAs, CDA1 and CDA2, have been identified as candidate molecules that may mediate gene conversion [19]. Generation of antibody diversity by gene conversion in birds, rabbits and cattle requires AID, which belongs to the apolipoprotein B mRNA editing enzyme, catalytic Bcl-w polypeptide family of molecules [26]. Phylogenetic analysis and secondary structure prediction suggest that AID and CDA1 are more closely related to each other than are AID and CDA2. Over-expression of the CDA1 molecule in yeast confers a mutagenic phenotype and increases the rate of intragenic recombination. Previous reports have revealed that CDA1 and CDA2 are expressed in VLRA+ and VLRB+ LLCs, respectively [27]. Thus, the jawless vertebrate CDA1 and CDA2 molecules may control gene conversion-like processes in VLRA+ and VLRB+ LLCs, respectively. Jawless vertebrates possess both soluble and membrane-bound forms of VLRB [17], [28]. Soluble VLRB antibodies are organized into pentamers or tetramers of dimers, similarly to immunoglobulin M of jawed vertebrates. The cysteine residues that are located in the 3′-invariant stalk region are required for VLRB antibodies to form oligomers.