C. Namikawa et al., ISOLATION OF XENOPUS LMP-7 HOMOLOGS - STRIKING ALLELIC DIVERSITY AND LINKAGE TO MHC, The Journal of immunology, 155(4), 1995, pp. 1964-1971
The mammalian low molecular mass protein-7 (LMP-7) gene resides in the
class II region of the MHC, and its product is most probably involved
, as a component of a proteasome, in the processing of Ags to be prese
nted by the MHC class I molecules. To elucidate the evolution of the L
MP-7 gene at both the primary structure and genetic levels, we isolate
d LMP-7 cDNA clones from amphibian Xenopus laevis, which last shared a
common ancestor with mammals 350 x 10(6) years ago. Two distinctive c
lones, showing an 85% predicted amino acid sequence identity with each
other and 69 to 72% identity with human and mouse LMP-7, were identif
ied from a liver cDNA library of outbred frogs and named XeLMP-7A and
XeLMP-7B. XeLMP-7A- and XeLMP-7B-specific probes were used to detect t
he corresponding genes by using partially inbred frogs with known MHC
haplotypes. DNA of the g and j haplotypes hybridized with the XeLMP-7A
probe, whereas the f and r haplotype DNA hybridized with the XeLMP-7B
probe. These hybridization patterns cosegregated with the MHC haploty
pes among offspring of an f/f x f/g cross, and one recombinant reveale
d that the LMP-7 gene is linked more closely to class II than to class
I or class III genes. Taken together, the data indicate that XeLMP-7A
and XeLMP-7B are highly diverse alleles at a single locus in the frog
MHC. The great allelic diversity can be explained either by coselecti
on with particular class I alleles or by differential silencing of MHC
genes in the polyploid X. laevis.