RESUMEN
Gram-negative bacteria are surrounded by two membranes. A special feature of the outer membrane is its asymmetry. It contains lipopolysaccharide (LPS) in the outer leaflet and phospholipids in the inner leaflet1-3. The proper assembly of LPS in the outer membrane is required for cell viability and provides Gram-negative bacteria intrinsic resistance to many classes of antibiotics. LPS biosynthesis is completed in the inner membrane, so the LPS must be extracted, moved across the aqueous periplasm that separates the two membranes and translocated through the outer membrane where it assembles on the cell surface4. LPS transport and assembly requires seven conserved and essential LPS transport components5 (LptA-G). This system has been proposed to form a continuous protein bridge that provides a path for LPS to reach the cell surface6,7, but this model has not been validated in living cells. Here, using single-molecule tracking, we show that Lpt protein dynamics are consistent with the bridge model. Half of the inner membrane Lpt proteins exist in a bridge state, and bridges persist for 5-10 s, showing that their organization is highly dynamic. LPS facilitates Lpt bridge formation, suggesting a mechanism by which the production of LPS can be directly coupled to its transport. Finally, the bridge decay kinetics suggest that there may be two different types of bridges, whose stability differs according to the presence (long-lived) or absence (short-lived) of LPS. Together, our data support a model in which LPS is both a substrate and a structural component of dynamic Lpt bridges that promote outer membrane assembly.
Asunto(s)
Membrana Externa Bacteriana , Proteínas Portadoras , Bacterias Gramnegativas , Lipopolisacáridos , Proteínas de la Membrana , Membrana Externa Bacteriana/química , Membrana Externa Bacteriana/metabolismo , Proteínas de la Membrana Bacteriana Externa/química , Proteínas de la Membrana Bacteriana Externa/metabolismo , Transporte Biológico , Proteínas Portadoras/química , Proteínas Portadoras/metabolismo , Escherichia coli/química , Escherichia coli/citología , Escherichia coli/metabolismo , Proteínas de Escherichia coli/química , Proteínas de Escherichia coli/metabolismo , Bacterias Gramnegativas/química , Bacterias Gramnegativas/citología , Bacterias Gramnegativas/metabolismo , Lipopolisacáridos/química , Lipopolisacáridos/metabolismo , Proteínas de la Membrana/química , Proteínas de la Membrana/metabolismoRESUMEN
In mononucleated cells, nuclear size scales with cell size, but does this relationship extend to multinucleated cells? In this issue of Developmental Cell,Windner et al. (2019) examine scaling of nuclei in multinucleated Drosophila muscle fibers and identify global and local cellular inputs that contribute to nuclear size regulation.
