Genetics, Structure, and Function of Histocompatibility Antigens 
western Medical Center at Dallas) to produce M3 
in amounts sufficient for a structural analysis. 
RMA-S Mutant Cells 
The heavy chains of MHC class I molecules are 
not stable in the properly folded conformation at 
body temperature unless they bind a peptide as 
well as the 182-microglobulin light chain. RMA-S 
mutant cells make MHC class I heavy chains and 
j82-microglobulin, but do not display them on 
their surface. The defect can be circumvented by 
adding suitable synthetic peptides to the mutant 
cells, which will then display those MHC mole- 
cules that bind the added peptide. Not only clas- 
sical MHC class I molecules and M3, which were 
already known to present peptides, are reduced 
or missing from these cells, but also other MHC 
class I molecules, such as Qa-1 . These molecules 
are recognized by antibodies or killer T lympho- 
cytes, but their physiological function is not yet 
clear. The RMA-S cell line shows that they all bind 
peptides. 
The RMA-S mutation affects one of two MHC 
genes whose protein products together make a 
channel to transport peptides from the cytoplasm 
into the endoplasmic reticulum, where MHC 
class I molecules fold. Surface display of all such 
molecules, including Mta and Qa-1, is restored 
when a normal version of this gene is expressed 
in the RMA-S cell line. This tells us that the mito- 
chondrial peptide is transported from the cyto- 
plasm into the endoplasmic reticulum by the 
same mechanism that other peptide antigens use. 
We have found that the drug oligomycin, which 
stimulates protein degradation in mitochondria, 
increases the surface display of both Mta (as ex- 
pected) and Qa-1 on RMA-S cells, suggesting that 
Qa-1 can also bind a mitochondrial peptide. We 
believe that the increased supply of these pep- 
tides floods the inefi'ective transport system and 
overcomes the defect. 
/?2-Microglobulin Polymorphism 
Mice are the only mammals in which allelic 
forms of /32-microglobulin have been described. 
Because of its intimate interaction with the pep- 
tide-binding parts of MHC class I heavy chains, an 
allelic difi'erence of 182-microglobulin that alters 
amino acid 85 from aspartic acid to alanine can 
subtly aff'ect the structure of MHC class I mole- 
cules and their ability to bind particular pep- 
tides. We have found four new alleles of (82- 
microglobulin among wild mice of the species 
Mus musculus. These differ by only one or two 
amino acids relative to the |82microglobulin of 
inbred mice. Three of these wild alleles have va- 
line in position 85 and may therefore alter pep- 
tide binding. 
We also looked at the |82-niicroglobulin gene 
from the species Mus spretus, which has been 
separated from Mws by about 1 million 
years of evolution. To our surprise, this (82- 
microglobulin differed from that of inbred mice 
by 12 amino acids and had only a single, silent 
mutation, which afi'ected the DNA sequence, but 
not the protein. For comparison, rat and mouse 
/32-microglobulins differ by 14 amino acids. We 
would have expected at least an equal number of 
silent mutations and amino acid changes for the 
spretus form. 
Such drastic divergence contrasts with the mod- 
est changes seen in other members of the immu- 
noglobulin superfamily and suggests that the 
molecule has been selected for its differences. 
The diversification may be part of the speciation 
process, but it may be limited to molecules 
that can tolerate change. All changes in the 
microglobulin molecule are in loops or on the 
face away from the MHC class I heavy chain, spar- 
ing the face that interacts with the conserved do- 
main of the heavy chains. 
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