Mutational Analysis of the Putative Lipid Transfer Protein YhdP in Escherichia coli

Abstract

The outer membrane (OM) of Gram-negative bacteria is an asymmetric bilayer–with lipopolysaccharide molecules in the outer leaflet and glycerophospholipids (PLs) in the inner leaflet–that confers antibiotic resistance. However, how PLs are transported from the inner membrane (IM) to the OM for assembly is poorly understood. To help elucidate this process, we investigated a protein in the IM, YhdP, that is hypothesized to act as a bridge that transport PLs from the IM to the OM. Using the Phyre2 structural prediction server, we identified regions of YhdP that are homologous to predicted lipid transport proteins (LTPs). We also discovered a CxxC motif, which is usually involved in redox functions and is found in redox enzymes, in YhdP. In order to explore YhdP function and examine whether this IM protein behaves like a bridge for intermembrane PL movement, we successfully generated loss-of-function mutations in the LTP-like domains, the transmembrane domain, and the CxxC motif of YhdP. We then screened for suppressors of these mutations and found that two mutations in the LTP-like domains are suppressed by decreasing the activity of MlaA. MlaA is an OM lipoprotein component of the Mla system, which functions to maintain the asymmetrical structure of the OM. Our data demonstrate that compromising the Mla system suppresses the defects of our yhdP mutations and supports a model where YhdP facilitates the transport of PLs to the OM from the IM.