A nontransportable substrate for lactose permease

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• Lactose permease (LacY) is a key transporter in Escherichia coli, crucial for lactose metabolism.
• Understanding LacY's transport mechanism requires specific substrate analogs to probe binding and translocation.
• Existing substrates do not fully elucidate the intricate steps of LacY-mediated transport.

Purpose of the Study:

• To synthesize and characterize a novel non-transported substrate for lactose permease.
• To investigate the binding affinity and transport kinetics of this synthetic peptide galactoside.
• To gain deeper insights into the structure-function relationship of lactose permease.

Main Methods:

• Chemical synthesis of 6'-[(N-phenylalanylphenylalanyl)amino]hexyl 1-thio-beta-D-galactoside.
• Competition binding assays using purified LacY in cytoplasmic membranes.
• Counterflow transport assays with reconstituted proteoliposomes containing purified LacY.

Main Results:

• The synthesized peptide galactoside exhibited high-affinity binding to lactose permease (Kd = 150 microM).
• Transport of the substrate was undetectable, with an upper limit for the rate constant of 0.02 s-1.
• The transport rate was significantly lower (3 orders of magnitude) compared to natural lactose.

Conclusions:

• The synthetic peptide galactoside acts as a high-affinity binder but a non-transporter for lactose permease.
• This finding highlights the distinct structural requirements for binding versus translocation in LacY.
• The study provides a valuable tool for dissecting the mechanism of sugar transport by LacY.