Bacterial long-chain fatty acid transport: Identification of amino acid residues within the outer membrane protein FadL required for activity

The outer membrane protein FadL (product of the fadL gene) of Escherichia coli is required for the specific binding and transport of exogenous long-chain fatty acids prior to metabolic utilization. The carboxyl end of FadL has been proposed to play a crucial role by facilitating the transport of long-chain fatty acids. In an attempt to define specific amino acid residues within carboxyl region of FadL essential for activity, a series of deletion and point mutations within the 3′ end of the fadL⁺ gene have been constructed and characterized. These fadL mutants were classified into three categories based on functional properties attributable to the altered FadL proteins: (i) those that had essentially wild-type levels of long-chain fatty acid binding and transport, (ii) those that had wild-type levels of long-chain fatty acid binding but were defective in transport, and (iii) those that were defective for both long-chain fatty acid binding and transport. These findings demonstrate that amino acid residues Phe⁴⁴⁸, Pro⁴²⁸, Val⁴¹⁰, and Ser³⁹⁷ are required for optimal levels of long-chain fatty acid transport and that amino acid residues Pro⁴²⁸ and Val⁴¹⁰ are essential for long-chain fatty acid binding.