Topological "frustration" in multispanning E. coli inner membrane proteins

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• The arrangement of E. coli inner membrane proteins is crucial for cellular function.
• Positively charged residues significantly influence protein topology within the cell membrane.

Purpose of the Study:

• To investigate the role of positively charged residues, specifically lysine, in determining the topology of E. coli inner membrane proteins.
• To explore how lysine distribution in connecting loops affects the insertion and orientation of transmembrane stretches.

Main Methods:

• Construction of model proteins with four potential transmembrane segments.
• Systematic placement of lysine residues in loops connecting transmembrane spans.
• Analysis of resulting protein topologies and membrane insertion patterns.

Main Results:

• Membrane insertion of individual helical hairpins is a locally determined process.
• Topologically frustrated proteins, with conflicting charge distributions, exhibit incomplete membrane spanning (e.g., "leave-one-out" topology).
• Only 3 out of 4 potential transmembrane stretches were observed to span the membrane in frustrated models.

Conclusions:

• The distribution of positive charges, especially lysine, is a primary determinant of E. coli inner membrane protein topology.
• Helical hairpin insertion occurs independently, simplifying the understanding of multi-spanning protein biogenesis.
• Understanding these insertion mechanisms provides insights into membrane protein evolution and biogenesis.