Abstract
Cell envelope biogenesis is an essential process that requires coordination of many complex pathways, including the synthesis of fatty acids, lipopolysaccharides, and glycerophospholipids. Loss of Escherichia coli YhcB has been demonstrated to result in filamentous cell morphology and membrane defects due to overactive fatty acid biosynthesis, a phenotype that intensifies in early stationary phase when lipid biosynthesis is normally downregulated. In bacteria, fatty acid biosynthesis is initiated by the acetyl-coenzyme A (CoA) carboxylase (ACC) complex, which catalyzes the carboxylation of acetyl-CoA to malonyl-CoA, the first and key regulatory step in the pathway. Here, we show that YhcB interacts with AccA, one of four subunits of the ACC complex. This interaction is growth-phase dependent, occurring specifically during the transition from exponential to stationary phase. We also report that AccA undergoes proteolytic degradation, a process that is modulated by YhcB, but not strictly dependent upon its presence. While YhcB is not required for AccA degradation, YhcB and AccA interactions correlate with reduced fatty acid biosynthesis in the early stationary phase. These findings indicate that YhcB may function by sequestering AccA, thereby inhibiting ACC activity. Together, our findings suggest two previously unrecognized mechanisms for regulating ACC activity: targeted proteolysis of AccA and its sequestration by YhcB. Importance: The gram-negative cell envelope is synthesized through coordination of many complex pathways, requiring adaptable regulation at the DNA, RNA, and protein levels. Fatty acid biosynthesis, a highly energy-demanding process, is essential for cell viability and a promising target for antimicrobial design. We identify two previously unrecognized mechanisms that regulate this pathway: proteolytic turnover and YhcB-mediated sequestration of a protein dedicated to the initiation of fatty acid biosynthesis. These findings reveal new layers of control over membrane biogenesis that could be exploited for antimicrobial strategies.