Interaction of alkali light chain 1 with actin: effect of ionic strength on the cross-linking of alkali light chain 1 with actin.

To determine the spatial relationship between alkali light chain and actin in the actosubfragment-1 complex, we studied the cross-linking of actin and subfragment-1 with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. We found that (a) alkali light chain 1 was cross-linked to actin at two sites in the extrapeptide region, and (b) cross-linking of these two sites, especially the one which was very close to the NH2 terminal of the alkali light chain, to actin was inhibited drastically when the KCl concentration was increased from 0 to 100 mM. Since the inhibition of cross-linking with carbodiimide reagent means separation of amino and carboxyl groups in alkali light chain and actin, we suggest that this decrease in electrostatic attraction is the reason why subfragment-1 with alkali light chain 1 has higher affinity to actin than subfragment-1 with alkali light chain 2 at low ionic strength but has almost the same affinity at moderate ionic strength.