Investigating the physiological roles of YqjA and YghB: members of the conserved DedA membrane protein family in Escherichia coli

Abstract

The aim of this dissertation is to characterize the physiological roles of E. coli YqjA and YghB, which belong to the ancient and conserved DedA protein family of hitherto unknown functions. YqjA and YghB are inner membrane (IM) proteins with 61% amino acid identity and share partially redundant functions. Such characterization is achieved by addressing the phenotypes exhibited by strain BC202 with in-frame deletions of yqjA and yghB. BC202 exhibits cell division defects, inability to grow at temperatures above 42°C and an altered phospholipid composition. The cell division defect of BC202 is attributed to the inefficient secretion of periplasmic amidases AmiA and AmiC by the twin arginine transport (Tat) pathway into the periplasm. This inefficient secretion by the Tat pathway is likely related to the loss of proton motive force (PMF) as demonstrated by fluorescence microscopy. In addition, several envelope stress response pathways are induced in BC202 under permissive growth conditions as demonstrated using lacZ fusions. The activation of the stress response pathways in BC202 is largely independent of its cell division and temperature sensitive phenotypes, and demonstrates compromised envelope integrity. The temperature sensitivity of BC202 is also related to the loss of PMF and is rescued by overexpression of the multidrug transporter MdfA or by growth in acidic media. The altered phospholipid composition of BC202 is unrelated to its cell division defect and temperature sensitivity and likely results from an adaptive response to compromised IM integrity. Together, these results suggest possible role(s) of YqjA/YghB in cellular PMF homeostasis and IM quality control either directly as proton transporters or indirectly as regulators of extant cellular mechanisms which participate in the homeostasis of PMF.