Cloning and characterization of Xenopus β2-microglobulin

cDNAs for Xenopus β2-microglobulin (β2m), the obligatory light chain of most vertebrate Major Histocompatibility Complex (MHC) class I molecules, were isolated and ESTs were identified. Alignment of the deduced amino acid sequence to other species' β2m showed that the overall structure is evolutionarily conserved, and phylogenetic analysis showed that the Xenopus β2m sequence is intermediate between fish and bird/mammal β2m. The Xenopus β2m mRNA is expressed ubiquitously with highest expression in intestine, spleen, and thymus, correlating well with classical class Ia expression. β2m mRNA and protein were also detected in Xenopus thymic tumor and kidney cell lines. Segregation analysis on a tetraploid Xenopus laevis family revealed two independently segregating, non-MHC-linked loci. As expected, only one locus was found in the diploid Xenopus tropicalis, strongly suggesting that the two β2m loci in the tetraploid species were generated by genome-wide duplication, and did not undergo diploidization unlike many other MHC genes.