Abstract131I‐labeled IgD‐like molecules extracted from the membrane of mouse spleen, and lymph node cells and their constitutive chains have been characterized by sodium dodecyl sulfate polyacrylamide slab gel electrophoresis (SDS‐PAGE) followed by radioautography. Unreduced mouse IgD‐like molecules concentrated in two definite bands (IgD1and IgD2). IgD, migrated slightly ahead of131I‐labeled mouse membrane IgM and displayed a mobility very similar to that of131I‐labeled IgD extracted from the membrane of human tonsil cells. IgD2migrated faster than both IgD1and human membrane IgD (mIgD), but slower than14C‐labeled mouse IgG2bused as marker. The different mobility of IgD1and IgD2was due to difference in size of their respective H‐chains (δ1and δ2), as shown by SDS‐PAGE analysis of the two molecules after reduction. Mol.wt. determination of IgD1, IgD2and of the corresponding constitutive chains are consistent with an H2L2structure. IgD1and IgD2were partially replaced following prolonged dialysis by molecules with a SDS‐PAGE mobility consistent with an HL structure, IgD3. In contrast to that found for spleen and lymph node, IgD1only was detected on the surface of bone marrow cells.Immunofluorescence experiments have shown that while the majority of mouse lymph node or spleen cells bear IgM and IgD, IgM was found to be the predominant Ig class on the surface of bone marrow cells, although a certain number of IgD‐IgM and a few IgD‐positive cells were found.Experiments where mouse lymphocytes have been treated with pronase have indicated that, in analogy with mlgD of primates, mouse IgD‐like molecules are very labile and can be removed from the cell surface by very
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