FIGURE 5.

C-terminal truncations of SMSs decrease homodimer formation. A, schematic of the N- and/or C-terminal truncations of SMS1 and SMS2. Cys⁵⁰ in SMS1 and Cys³⁴³/Cys³⁴⁸ in SMS2 are represented as C in the circle and were identified as sites cross-linked by cysteine cross-linking reagents (Fig. 2). The boxes represent transmembrane (TM) segments TM1 to TM6 as indicated. All constructs contain a FLAG or V5 tag at the C terminus. Both the C-terminal deletion mutants SMS1-ΔC22 and SMS2-ΔC30 are expected to have 44-amino acid extensions from the end of TM6. B–E, COS7 cells were transfected with a plasmid encoding V5-tagged SMS1-WT, SMS1-ΔN68, SMS1-ΔC22, or SMS1-ΔN68C22 (B and C) or V5-tagged SMS2-WT or SMS2-ΔC30 (D and E). 24 h post-transfection, the membrane fractions were separated by BN-PAGE and immunoblotted with anti-V5 antibody (B and D). The monomer-to-homodimer ratio was determined from the intensities of monomer and homodimer in the immunoblot assay (C and E). Individual data points are shown as a scatterplot. Values represent the mean ± S.D. from at least three independent experiments. *, p < 0.01.