Fig. 5. Direct interaction between SOR-1 and SOP-2. (A) GST-SOP-2 fusion proteins were incubated with ³⁵S-labeled SOR-1 (amino acids 1-503) to map the interaction domain in SOP-2. Only the N-terminal domain (amino acids 1-217) of SOP-2 bound SOR-1. Smaller fragments of SOP-2 were used to further map the minimal interaction domain. (B) The N terminus of SOR-1 interacts with SOP-2. GST-SOR-1 fusion proteins were incubated with ³⁵S-labeled SOP-2. Twenty percent of the ³⁵S-labeled protein used in the binding reactions was shown. No interaction was detected between the ³⁵S-labeled protein and GST alone or GST fused to the C terminus of SOR-1 but binding was detected between SOP-2 and the N terminus of SOR-1. Smaller fragments of SOR-1 were used to map the interaction domain further. (C) Schematic representation of protein interaction domains of SOR-1 and SOP-2 (connected by arrow). SOP-2 also contains a protein interaction SAM domain, which mediates its self-association (Zhang et al., 2003). (D) Interaction between N terminus of SOP-2 (residues 58-140) and SOR-1 (residues 48-200). The chromatogram of gel filtration is shown on the top and the fractions from the complex are shown at the bottom. Q indicates proteins purified through ion exchange column.