Independent Membrane Binding Properties of the Caspase Generated Fragments of the Beaded Filament Structural Protein 1 (BFSP1) Involves an Amphipathic Helix

Background: BFSP1 (beaded filament structural protein 1) is a plasma membrane, Aqua- 21 porin 0 (AQP0/MIP)-associated intermediate filament protein expressed in the eye lens. BFSP1 is 22 myristoylated, a post-translation modification that requires caspase cleavage at D433. Bioinformatic 23 analyses suggested that the sequences 434-452 were a-helical and amphipathic. Methods and Re- 24 sults: By CD spectroscopy, we show that the addition of trifluoroethanol induced a switch from an 25 intrinsically disordered to a more a-helical conformation for the residues 434-467. Recombinantly 26 produced BFSP1 fragments containing this amphipathic helix bind to lens lipid bilayers as deter- 27 mined by surface plasmon resonance (SPR). Lastly, we demonstrate by transient transfection of non- 28 lens MCF7 cells that these same BFSP1 C-terminal sequences localise to plasma membranes and to 29 cytoplasmic vesicles. These can be co-labelled with the vital dye, lysotracker, but other cell compart- 30 ments such as the nuclear and mitochondrial membranes were negative. The N-terminal myristoy- 31 lation of the amphipathic helix appeared not to change either the lipid affinity or membrane locali- 32 sation of the BFSP1 polypeptides or fragments we assessed by SPR and transient transfection, but it 33 did appear to enhance its helical content. Conclusions: These data support the conclusion that C- 34 terminal sequences of human BFSP1 distal to the caspase site at G433 have independent membrane 35 binding properties via an adjacent amphipathic helix

An amphipathic helix facilitates the membrane binding properties of BFSP1 and its caspase-generated C-terminal domain

Background: BFSP1 (beaded filament structural protein 1) is a plasma membrane, Aqua- 21 porin 0 (AQP0/MIP)-associated intermediate filament protein expressed in the eye lens. BFSP1 is 22 myristoylated, a post-translation modification that requires caspase cleavage at D433. Bioinformatic 23 analyses suggested that the sequences 434-452 were a-helical and amphipathic. Methods and Re- 24 sults: By CD spectroscopy, we show that the addition of trifluoroethanol induced a switch from an 25 intrinsically disordered to a more a-helical conformation for the residues 434-467. Recombinantly 26 produced BFSP1 fragments containing this amphipathic helix bind to lens lipid bilayers as deter- 27 mined by surface plasmon resonance (SPR). Lastly, we demonstrate by transient transfection of non- 28 lens MCF7 cells that these same BFSP1 C-terminal sequences localise to plasma membranes and to 29 cytoplasmic vesicles. These can be co-labelled with the vital dye, lysotracker, but other cell compart- 30 ments such as the nuclear and mitochondrial membranes were negative. The N-terminal myristoy- 31 lation of the amphipathic helix appeared not to change either the lipid affinity or membrane locali- 32 sation of the BFSP1 polypeptides or fragments we assessed by SPR and transient transfection, but it 33 did appear to enhance its helical content. Conclusions: These data support the conclusion that C- 34 terminal sequences of human BFSP1 distal to the caspase site at G433 have independent membrane 35 binding properties via an adjacent amphipathic helix