Competitive binding of α18-21, α18-21¹⁸⁵⁷, or α18-21^1857-Δ46 to GSTβ1-4⁺. In each assay, 250 pmol of ³⁵S α18-21 and 250 pmol of GSTβ1-4⁺ were mixed with 0, 125, 250, 500, and 1,000 pmol of each competitor (α18-21, α18-21¹⁸⁵⁷, or α18-21^1857-Δ46) in a final volume of 400 μL in PBS/0.1% bovine serum albumin for 30 minutes on ice. Bound complexes were sedimented with glutathione Sepharose 4B in a Millipore 0.22-μm filtration unit and the filtrate was counted as the unbound fraction. The resin was washed twice and bound complexes were eluted with two aliqouts of 400 μL 0.2% SDS. Background corrected values for the amount of ³⁵S α18-21 from three different experiments were averaged (error bars show standard deviation). () α18-21; () α18-21¹⁸⁵⁷; (▪) α18-21^1857-Δ46.