rules out that it is Hmt. The Mbl probe cross-hy- 
bridizes with a second fragment that also was 
mapped to the Hmt region; this could represent a 
sixth class I gene in that region. 
C3R1 was cloned as a 5 kb fcoRI fragment, con- 
taining the 3' end of a class I gene. With a probe 
derived from this clone, two cosmids with the com- 
^ plete gene have been isolated, one of which con- 
tains an additional, intact class I gene, CRW2. Since 
C3R1 has a stop codon in the third exon, and no 
mRNA was detected with an exon 3 probe, it is a 
poor candidate for Hmt. CRW2 is interesting in that 
exons 2 and 3 differ strikingly from all other class I 
genes (although the glycosylation site in the al and 
the two cysteines in the al domain are conserved), 
whereas exon 4 shows a normal level of similarity 
and the gene encodes a proper transmembrane re- 
gion and a short cytoplasmic tail. CRW2 appears to 
be an intact class I gene, but it is not yet known 
whether it is expressed. 
C. A candidate for Hmt. R4B2 was first cloned from 
Hmt'° DNA as a 17 kb Bghl fragment. The homo- 
logue from Hmt^ mice has since been isolated in 
the form of one cosmid and several cDNA clones. 
The amino acid sequence of R4B2 shows about 
15%, 65%, 55%, 80%, and 30% similarity to known 
class I genes for exons 1, 2, 3, 4, and 5, respectively. 
It has a single glycosylation site at residue 86 in the 
al domain, a hydrophobic transmembrane domain, 
and a cytoplasmic tail of eight amino acids. R4B2 
mRNA is easily detectable in thymus, spleen, lymph 
nodes, liver, and kidney. 
The properties of R4B2 are consistent with what 
is known about Hmt. R4B2 shares a number of con- 
served, structurally important residues and has suf- 
ficient sequence similarity to other MHC class I 
genes to suggest that it would fold in the same 
manner. No charged residues point directly into the 
peptide-binding site modeled after the HLA-A2 
structure. 
III. T Cell Receptor for Mta. 
Recent reports have suggested that 7/8 T cell 
receptors (TCR) recognize MHC molecules en- 
coded in the distal end of the complex. Rabbit anti- 
TCRa and anti-TCR7 sera were therefore used to 
immunoprecipitate the surface-labeled TCR of a 
number of cytotoxic T cell lines specific for Mta. All 
cells gave a strong band with anti-TCRa and none 
with anti-TCR-y, showing that they all use a/p-re- 
ceptors. 
lY ^ewH-21& Mutant. 
Not all changes in MHC molecules result in 
strong T cell immune responses. A spontaneous 
mutant was identified among the progeny of a 
backcross of R4-1 to C57BL/6J, because it had lost 
the epitope of the H-2K^' molecule recognized by 
monoclonal antibody B8-24-3. Whereas the muta- 
tion has affected the binding of several monoclonal 
antibodies, whose epitopes have been mapped to 
the loop area of transition from a-helix to P-sheet 
around residues 77-90 of the al domain, the mu- 
tation does not affect T cell recognition of the 
H-2K'' molecule. The mutant does not reject skin 
from the parental strain, and there is no T cell pro- 
liferation or killing induced in vitro between mu- 
tant and parent. In a collaboration with the labora- 
tory of Dr. Larry Peace (Mayo Clinic), the mutant 
gene was sequenced and the mutation identified as 
a lysine to alanine transition in residue 89. 
Dr. Fischer-Lindahl is also Associate Professor of 
Microbiology and Biochemistry at the University of 
Texas Southwestern Medical Center at Dallas. 
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