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Clinical Pathology Department, Clinical Center, National Institutes of Health, Bethesda, MD 20892; and
Department of Medicine, Duke Medical Center, Durham, NC 27710
Mice infected with Listeria monocytogenes (LM)
generate CD8 effectors specific for f-MIGWII, the amino terminus of the
bacterial product lemA presented by the class Ib MHC molecule H2
M3wt. lemA has several distinctive properties: 1) it is
readily presented as an exogenous Ag in the absence of bacterial
infection; 2) it is processed by a TAP-independent pathway, which is
sensitive to chloroquine, pepstatin, and brefeldin; and 3) the
immunogenic portion of the molecule is extremely resistant to
proteolytic degradation even by proteinase K. To assess the structural
basis for these findings, we expressed a truncated variant (t-lemA)
containing the amino-terminal hexapeptide and the subsequent 27 amino
acids linked to a histidine tail in Escherichia coli,
and purified the product by affinity chromatography. Purified t-lemA
could be presented to f-MIGWII-specific effectors by macrophages and
fibroblasts at 110 nM. Unlike f-MIGWII, which binds directly to H2
M3wt, t-lemA required processing by a chloroquine-,
pepstatin-, and brefeldin-sensitive pathway. Brefeldin sensitivity
often implies endogenous processing in the cytoplasm, but several lines
of evidence suggest translocation to the cytoplasm and proteosomal
degradation are not critical for t-lemA presentation. Unlike f-MIGWII,
t-lemA was profoundly resistant to proteinase K, and, using
35S-labeled t-lemA, we could identify the region from
position 1 to
30 as the protease-resistant element. Thus, the
hydrophobic peptide sequence following f-MIGWII can account for the
unusual properties of lemA noted above. Analogous modification could be
used to alter the properties of other peptide Ags presented by class I
MHC products.
This article has been cited by other articles:
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