Dissecting the Binding Interface of the Septin Polymerization Enhancer Borg BD3

Author(s)

Castro, Danielle K. S. V. ; Rosa, Higor V. D. ; Mendonc, Deborah C. ; Cavini, Italo A. ; Araujo, Ana P. U. ; Garratt, Richard C.

• Document type: Journal article
• Source: Journal of Molecular Biology; v. 435, n. 13, p. 16-pg., 2023-05-10.

Abstract

The molecular basis for septin filament assembly has begun to emerge over recent years. These filaments are essential for many septin functions which depend on their association with biological membranes or components of the cytoskeleton. Much less is known about how septins specifically interact with their binding partners. Here we describe the essential role played by the C-terminal domains in both septin polymerization and their association with the BD3 motif of the Borg family of Cdc42 effector proteins. We provide a detailed description, at the molecular level, of a previously reported interaction between BD3 and the NC-interface between SEPT6 and SEPT7. Upon ternary complex formation, the heterodi-meric coiled coil formed by the C-terminal domains of the septins becomes stabilized and filament forma-tion is promoted under conditions of ionic strength/protein concentration which are not normally permissible, likely by favouring hexamers over smaller oligomeric states. This demonstrates that binding partners, such as Borg's, have the potential to control filament assembly/disassembly in vivo in a way which can be emulated in vitro by altering the ionic strength. Experimentally validated models indicate that the BD3 peptide lies antiparallel to the coiled coil and is stabilized by a mixture of polar and apolar con-tacts. At its center, an LGPS motif, common to all human Borg sequences, interacts with charged residues from both helices of the coiled coil (K368 from SEPT7 and the conserved E354 from SEPT6) suggesting a universal mechanism which governs Borg-septin interactions.(c) 2023 Elsevier Ltd. All rights reserved. (AU)

• FAPESP's process: 14/15546-1 - Septins: comparative studies and the correlation between structure and function
• Grantee: Richard Charles Garratt
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 18/20209-5 - Structural studies on the assembly of septin complexes by transmission electron microscopy and single particle analysis
• Grantee: Deborah Cezar Mendonça
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 18/19992-7 - Structural studies of septin heteromeric coiled-coils by nuclear magnetic resonance spectroscopy
• Grantee: Italo Augusto Cavini
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 20/02897-1 - Septin filaments: structure, polymerization and role in pathologies
• Grantee: Richard Charles Garratt
• Support Opportunities: Research Projects - Thematic Grants