Understanding Bacterial Outer Membrane Transport

The question asks to identify the specific type of protein responsible for transporting small molecules across the bacterial outer membrane. These proteins are described as trimeric integral membrane proteins and facilitate the passage of molecules smaller than approximately 600 daltons.

Identifying the Transport Protein

Let's analyze the options:

• Siderophores: These are small, high-affinity iron-chelating compounds secreted by microorganisms. They are involved in iron uptake but are not the integral membrane proteins described.
• ABC transporters: These are large membrane protein complexes that use ATP hydrolysis to move molecules across membranes, typically involved in larger molecules or ions, and don't fit the description of facilitating small molecule passage through passive channels.
• Porins: These are β-barrel proteins that form pores in the outer membrane of Gram-negative bacteria. They are typically trimeric and allow the passive diffusion of small, hydrophilic molecules (like nutrients and ions up to about 600 daltons) across the outer membrane. This matches the question's description perfectly.
• Hopanoids: These are lipid compounds found in bacterial cell membranes, structurally similar to cholesterol, involved in membrane fluidity but not in transport.

Conclusion on Transport Proteins

Based on the function and structure described (trimeric integral membrane proteins facilitating transport of molecules < 600 daltons across the outer membrane), porins are the correct identification.