Porin from bacterial and mitochondrial outer membranes

Area of Science:

• Microbiology
• Cell Biology
• Evolutionary Biology

Background:

• Gram-negative bacteria possess outer membranes that function as molecular filters.
• These filters have specific exclusion limits for hydrophilic substances, varying by bacterial species.
• Porins, major outer membrane proteins, form pores responsible for this molecular sieving.

Purpose of the Study:

• To investigate the molecular sieving properties of bacterial outer membranes.
• To explore the role of porins in determining the exclusion limit.
• To examine the outer membrane of mitochondria in light of their proposed bacterial ancestry.

Main Methods:

• Analysis of molecular weight exclusion limits in different bacterial species (e.g., Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa).
• Characterization of porin structure and pore dimensions within bacterial outer membranes.
• Comparison of mitochondrial outer membrane permeability with bacterial outer membranes.

Main Results:

• Bacterial outer membrane exclusion limits range from 600-800 Daltons for enteric bacteria to 6000 Daltons for Pseudomonas aeruginosa.
• Porins form trimers with pores of 1.2-2 nm diameter, dictating membrane permeability.
• Mitochondrial outer membranes exhibit large pores permeable to molecules up to 6000 Daltons, supporting their bacterial origin hypothesis.

Conclusions:

• The outer membrane's molecular filtering capacity is primarily determined by porin structure and abundance.
• Mitochondrial outer membranes share functional similarities with gram-negative bacterial outer membranes, reinforcing the endosymbiotic theory.
• Mitochondrial porins, processed by the cell, may be regulated by electrical fields and metabolic processes.