Membrane Transport Proteins: New *E. coli* Analysis Challenges Standard Classification

A research team has proposed a new structural model for LetAB, a complex responsible for maintaining the outer envelope of Escherichia coli. Historically, the characterisation of membrane transport proteins has been hindered by their hydrophobic nature and resistance to crystallisation; while a few dominant families explain most known processes, a vast number of predicted transporters have remained structurally undefined. This study attempts to fill that void by applying modern predictive modelling to the LetAB complex.

Technical contrast: Sequence markers vs structural folding

The methodology highlights a significant divergence between traditional genomic analysis and modern structural prediction. Previously, researchers relied heavily on linear genetic markers—specific sequences of amino acids—to categorise proteins into families. If a protein lacked these sequence markers or displayed unusual composition (often correlated with variations in genomic GC content), it defied classification. The approach used here bypasses reliance on linear sequence markers. By utilising AlphaFold predictions alongside deep mutational scanning, the team assessed the protein based on its 3D architecture rather than its genetic script. This shift reveals that while LetA shares no sequence identity with known families, its folded structure is unmistakably similar to eukaryotic tetraspanins. Reliance on traditional sequence markers effectively created a blind spot, obscuring evolutionary links that only structural analysis could detect.

Re-evaluating **membrane transport proteins** classification

The LetAB complex localises to the inner membrane. The data indicates that LetA acts as a loader, poised to transfer lipids into its partner, LetB. LetB is described as an MCE protein that forms a tunnel approximately 225 Å in length, bridging the cell envelope. This tunnel mechanism represents a significant departure from the pump-and-channel models typically associated with bacterial transport. The authors combined these structural findings with molecular dynamics simulations to propose a continuous transport model.

Perhaps the most contentious finding is the evolutionary implication. The LetA transmembrane domains adopt a fold related to the eukaryotic tetraspanin family. This family includes transmembrane AMPA receptor regulatory proteins (TARPs) and claudins, which are essential for mammalian neural transmission and tissue barriers. Finding such an architecture in a bacterial phospholipid transporter suggests that the structural lineage of these proteins is far older than previously thought. It implies that the machinery for complex signalling in mammals may have originated from basic bacterial maintenance systems.

While the structural data is robust, the functional model relies heavily on simulations. Molecular dynamics provide a theoretical view of how lipids might move through the LetB tunnel, but this is distinct from observing the movement in real-time. The study successfully maps the static architecture, yet the precise kinetic efficiency of this 'tunnel' compared to traditional ATP-driven pumps remains a subject for further biochemical verification.