A spin label study of the ionic strength dependent conformational change in the human Ia molecule

Purified human Ia molecules were labelled with maleimide or isothiocyanate spin labels or by reacting "TEMPAMINE" spin label with the neuraminic acid of their carbohydrate residues. It was found that increasing the ionic strength from 0.05 to 0.75 markedly increased the dipolar interaction between the maleimide-attached labels, but no effect was found of ionic strength or motion on dipolar interaction with the other two labels. The effect of increasing ionic strength could be blocked by the prior addition of Ia-specific antibody, but could not be reversed by the addition of antibody after ionic strength was increased. These findings complement an earlier finding that increasing ionic strength over the range 0.05-0.75 has an inhibitory effect on the combination of Ia with its antibody. Because the maleimide spin labels attach predominantly to SH groups it is suggested that increasing ionic strength causes conformational changes in the immunoglobulin loop region which alter the accessibility of the Ia antibody-binding site.