Length matters: the disordered N-terminus of Pal coordinates Lpp exclusion for outer membrane constriction in E. coli

The peptidoglycan-associated lipoprotein (Pal) is essential for outer membrane integrity in gram-negative bacteria, but how its N-terminal disordered region contributes to cell division remains unclear. Here, we truncated this region in Escherichia coli (pal^-8: 8-aa deletion, pal^-24: 24-aa deletion) and found that while neither truncation impaired Pal localization or growth, both mutants exhibited graded sensitivity to membrane stressors (1 mM EDTA/1% SDS), with pal^-24 showing intermediate susceptibility between pal^-8 and Δpal. Atomic force microscopy revealed that the disordered region length fine-tunes Pal's ability to exclude Braun's lipoprotein (Lpp) from division sites: pal^-8 maintained wild-type Lpp-exclusion zones, whereas pal^-24 exhibited either absent or narrowed grooves. These results suggest that the disordered region acts as a length-dependent molecular spacer, ensuring efficient Pal-Lpp spatial exclusion during cell division. Our work establishes a mechanistic paradigm by which Pal's flexible N-terminus coordinates outer membrane constriction through dynamic competition with Lpp, offering new insights into the functional plasticity of disordered domains in bacterial morphogenesis.IMPORTANCEThe outer membrane of bacteria like E. coli must constrict during division, but the full mechanism is unclear. We studied peptidoglycan-associated lipoprotein (Pal), a protein that helps coordinate this process. Pal has a disordered linker region, and we found that the length of this linker acts as a molecular ruler. Shortening the linker disrupts Pal's ability to properly organize the division site and create a zone required for outer membrane remodeling. This reveals how a seemingly unstructured protein region can perform a precise, measurement-dependent function. Since disordered linkers are common in bacterial outer membrane lipoproteins, our work suggests a general design principle for fine-tuning cell envelope dynamics during growth and division.