Skp Trimer Formation Is Insensitive to Salts in the Physiological Range

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• The seventeen kilodalton protein (Skp) is a chaperone essential for the biogenesis of the Gram-negative outer membrane.
• Skp facilitates the transport and insertion of unfolded outer membrane proteins (uOMPs).
• Current models propose that Skp functions as a trimer to bind its uOMP substrates.

Purpose of the Study:

• To investigate the oligomeric state of Skp under physiological conditions.
• To determine the role of Skp oligomerization in outer membrane protein biogenesis.
• To challenge the existing model of Skp trimerization.

Main Methods:

• Sedimentation equilibrium analysis was employed to determine the oligomeric state of Skp.
• Van't Hoff analysis was used to study Skp self-association thermodynamics.
• The influence of various ions (Na+, Cl-, Mg2+, PO43-) on Skp trimerization was assessed.

Main Results:

• Skp is not an obligate trimer under physiological conditions.
• Ion concentrations (Na+, Cl-, Mg2+, PO43-) do not influence Skp trimerization, suggesting negligible electrostatic involvement in Skp assembly.
• Skp monomers are present at biologically relevant concentrations.
• Kinetic complex formation between Skp and uOMPs likely involves monomer assembly around the substrate.
• Thermodynamic analysis does not support a coupled folding and trimerization model for Skp.

Conclusions:

• The current model of Skp functioning exclusively as a trimer is inaccurate.
• Skp likely functions as a monomer or in dynamic equilibrium with monomers during uOMP binding.
• This finding necessitates a re-evaluation of the mechanism of outer membrane protein biogenesis mediated by Skp.