Calreticulin recognizes misfolded HLA-A2 heavy chains.
Our studies investigated functional interactions between calreticulin, an endoplasmic reticulum chaperone, and major histocompatibility complex (MHC) class I molecules. Using in vitro thermal aggregation assays, we established that calreticulin can inhibit heat-induced aggregation of soluble, peptide-deficient HLA-A2 purified from supernatants of insect cells. The presence of HLA-A2-specific peptides also inhibits heat-induced aggregation. Inhibition of heat-induced aggregation of peptide-deficient HLA-A2 by calreticulin correlates with a rescue of the HLA-A2 heavy chain from precipitation, by forming high-molecular-weight complexes with calreticulin. Complex formation between HLA-A2 heavy chains and calreticulin occurs at 50 degrees C but not 37 degrees C, suggesting polypeptide-based interactions between the HLA-A2 heavy chain and calreticulin. Once complexes are formed, the addition of peptide is not sufficient to trigger efficient assembly of heavy chain/beta2m/peptide complexes. Using a fluorescent peptide-based binding assay, we show that calreticulin does not enhance peptide binding by HLA-A2 at 37 degrees C. We also show that calreticulin itself is converted to oligomeric species on exposure to 37 degrees C or higher temperatures, and that oligomeric forms of calreticulin are active in inhibiting thermal aggregation of peptide-deficient HLA-A2. Taken together, these results suggest that calreticulin functions in the recognition of misfolded MHC class I heavy chains in the endoplasmic reticulum. However, in the absence of other endoplasmic reticulum components, calreticulin by itself does not enhance the assembly of misfolded MHC class I heavy chains with beta2m and peptides.
Mancino
Laura
L
Department of Microbiology and Immunology and Immunology Graduate Program, University of Michigan Medical School, Ann Arbor, MI 48109-0620, USA.
Rizvi
Syed Monem
SM
Lapinski
Philip Edward
PE
Raghavan
Malini
M
D000818:Animals; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D037282:Calreticulin; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000378:metabolism; D015789:HLA-A2 Antigen / Q000737:chemistry*; D006801:Humans; D007313:Insecta / Q000737:chemistry; D002679:Pan troglodytes; D010455:Peptides / Q000737:chemistry; D011233:Precipitin Tests; D011485:Protein Binding; D017510:Protein Folding; D012261:Ribonucleoproteins / Q000737:chemistry / Q000378:metabolism*; D013696:Temperature; D013997:Time Factors
D002135:Calcium-Binding Proteins; D037282:Calreticulin; D015789:HLA-A2 Antigen; D010455:Peptides; D012261:Ribonucleoproteins