The C-terminal region of peripherin/rds contains three predicted α-helical domains. One of these domains, corresponding to amino acids 311–322, form an amphiphilic α-helix previously shown to promote membrane fusion. The present studies were conducted to determine how the additional α-helical regions of the peripherin/rds C-terminus affect complex formation with rom-1, glycosylation, intracellular localization and membrane fusion properties. Bovine peripherin/rds and rom-1 were epitope tagged with an amino-terminal FLAG-tag or amino-terminal hemagglutinin (HA)-tag, respectively, and cloned into the pCI-neo expression vector for transient transfection into COS cells. Similarly, four C-terminal peripherin/rds truncation mutants (Δ1, Δ2, Δ3 and Δ4), corresponding to deletions of −19, −29, −39 and −59 amino acids were designed to disrupt the α-helical domains. Immunofluorescence microscopy and enzymatic digestions demonstrated that full-length peripherin/rds and the four C-terminal deletion mutants were localized to intracellular membranes and were all Endo-H sensitive. Western blotting and immunoprecipitation studies showed that the FLAG-tagged bovine peripherin/rds (full-length) was expressed as a 76 kDa dimer, which associates with HA-tagged rom-1 to form a higher order complex. The deletion mutants were also able to associate with rom-1. However, when analyzed using non-denaturing tricine electrophoresis, full-length peripherin/rds and the Δ1, Δ2 and Δ3 mutants formed homo-oligomeric complexes, while the Δ4 mutant appeared to form only homodimers suggesting a region upstream of amino acid 300 may be involved in C-terminal interactions. Membrane fusion was then evaluated using fluorescence resonance energy transfer (RET) techniques. Intracellular COS cell membranes containing full-length peripherin/rds fused with rod outer segment plasma membrane vesicles. This fusion was inhibited with the addition of a synthetic peptide (PP-5) corresponding to the fusion domain of peripherin/rds. In contrast, fusion was negligible with any of the C-terminal truncation mutants. Collectively, these results suggest that in addition to the fusion domain, other regions of the peripherin/rds C-terminus are required for fusion. Most interesting is the observation that the last 19 amino acids, a region downstream of the fusion peptide that is deleted in the Δ1 mutant, appear to be necessary for fusion. This region corresponds to the epitope for anti-peripherin/ rds monoclonal antibody 2B6, which is shown to partially inhibit peripherin/rds mediated membrane fusion
