Leaderless polypeptides efficiently extracted from whole cells by osmotic shock

Area of Science:

• Molecular Biology
• Cell Biology
• Protein Localization

Background:

• Protein translocation across cellular membranes typically relies on signal sequences.
• The periplasmic compartment in Escherichia coli is a common destination for secreted proteins.
• The mechanisms governing protein localization in the absence of signal sequences are not well understood.

Purpose of the Study:

• To investigate the intracellular localization of bacterial (DsbA, DsbC, rotamase) and mammalian (IL-1 receptor antagonist) proteins lacking their native signal sequences in E. coli.
• To identify the mechanism responsible for the localization of these leaderless proteins.
• To determine if this localization mechanism is conserved across species.

Main Methods:

• Site-directed mutagenesis to remove signal sequences from target proteins.
• Overexpression of leaderless proteins in E. coli.
• Cell fractionation and extraction techniques to isolate proteins.
• Biochemical assays (e.g., beta-galactosidase complementation) and genetic analysis (secA, ftsZ dependence).

Main Results:

• Leaderless DsbA, DsbC, rotamase, and IL-1 receptor antagonist accumulated in an osmotically sensitive cellular compartment.
• These leaderless proteins were extractable using methods for periplasmic proteins but were not associated with membranes.
• Localization was independent of the secA and ftsZ transport pathways, indicating a novel mechanism.

Conclusions:

• A signal sequence-independent mechanism facilitates the translocation of specific leaderless proteins to a unique cellular compartment in E. coli.
• Structurally unrelated proteins, including a mammalian protein, are recognized by this bacterial localization system.
• The findings suggest a conserved, evolutionarily broad mechanism for intracellular protein targeting.