Effect of avidin binding to SH1 on the interface between subfragment-1 and F-actin

The subfragment-1-avidin complex, in which avidin is attached to a well defined thiol group called SH1, was purified by CM cellulose column chromatography or affinity chromatography using lipoic acid agarose. The interaction of the purified complex with F-actin was compared to that of normal subfragment-1 using chemical cross-linking and limited tryptic digestion techniques. It was found that the binding of avidin to SH1 lowered the extent of cross-linking between the subfragment-1 heavy chain and actin. The amount of the 175K product decreased to about 50% of the normal level and that of the 165K product decreased to about 35%. It was also found that the binding of avidin abolished the protective effect of F-actin on the 50K-22K junction of the S-1 heavy chain against tryptic attack. Since more than 95% of the S-1-avidin complex was attached to F-actin under our experimental conditions, these changes are due to an alteration of the S-1-actin interface. Considering the facts that SH1 is located on the side of S-1 facing the F-actin, in the tertiary structure, and is close to the cross-linked site and to the 50K-22K junction, in the primary structure, it is quite likely that avidin bound to SH1 causes these effects by sterically preventing the close contact of S-1 and actin.