SecB-mediated protein export need not occur via kinetic partitioning

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• SecB is a cytosolic chaperone in Escherichia coli crucial for selective protein translocation via the Sec pathway.
• SecB binds unfolded precursor proteins but not native proteins, suggesting a role in substrate recognition.
• The kinetic partitioning model proposes that SecB selects substrates based on their folding rates.

Purpose of the Study:

• To quantitatively validate the kinetic partitioning model for SecB-mediated protein export.
• To elucidate the precise mechanisms governing SecB's selectivity for protein substrates.
• To investigate the interplay between protein folding, SecB binding, and interaction with other secretory components.

Main Methods:

• Independent measurement of rate constants for SecB interaction with pre-MBP and mature MBP.
• In vitro binding assays to assess SecB-substrate interactions.
• Kinetic analysis of protein folding and association with SecB.

Main Results:

• Measured rate constants accurately predicted substrate folding kinetics across varying concentrations.
• Kinetic partitioning is unlikely to be the primary mechanism for SecB-mediated export for many substrates.
• Substrate folding while bound to SecB and interaction with SecA emerged as critical factors.

Conclusions:

• The kinetic partitioning model does not fully explain SecB's role in protein export.
• Continued folding of SecB-bound substrates can inhibit export.
• Interaction with SecA promotes protein export, acting as a counteracting force.