Deciphering the rules governing assembly order of mammalian septin complexes

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Septins are conserved GTP-binding proteins involved in forming diffusion barriers and molecular scaffolds.
• Mammalian genomes encode 9-17 septin genes, producing ubiquitous and tissue-specific isoforms.
• The assembly mechanisms and native heteromeric states of mammalian septins are not well understood.

Purpose of the Study:

• To investigate the assembly states of mammalian septins.
• To elucidate the mechanisms directing the ordered assembly of septins into heteromeric units.
• To understand the role of homology subgroups in septin assembly.

Main Methods:

• Analysis of septin system in cells with altered septin expression (knockout/overexpression).
• Hydrodynamic analysis to determine septin complex size and shape.
• Single-particle analysis to visualize septin complex architecture.

Main Results:

• Septin assembly is interdependent and correlates with homology subgroups.
• Individual septins exist in stable six- to eight-subunit core heteromers containing SEPT2, SEPT6, and SEPT7 subgroup members.
• Heteromers with more than six subunits incorporate SEPT9.
• A model for homology subgroup-directed temporal assembly order is proposed.
• Native heteromers are rarely perfect palindromes due to multiple expressed isoforms.

Conclusions:

• Homology subgroup membership dictates the temporal order of septin assembly.
• This ordered assembly results in specific subunit arrangements within native heteromers.
• The complexity of mammalian septin expression leads to diverse, non-palindromic heteromeric structures.