SUMMARY How phospholipids are trafficked between the bacterial inner and outer membranes through the hydrophilic space of the periplasm is not known. We report that members of the mammalian cell entry (MCE) protein family form hexameric assemblies with a central channel capable of mediating lipid transport. The E. coli MCE protein, MlaD, forms a ring associated with an ABC transporter complex in the inner membrane. A soluble lipid-binding protein, MlaC, ferries lipids between MlaD and an outer membrane protein complex. In contrast, EM structures of two other E. coli MCE proteins show that YebT forms an elongated tube consisting of seven stacked MCE rings, and PqiB adopts a syringe-like architecture. Both YebT and PqiB create channels of sufficient length to span the periplasmic space. This work reveals diverse architectures of highly conserved protein-based channels implicated in the transport of lipids between the membranes of bacteria and some eukaryotic organelles.

Assembly of the trilaminar eggshell and development of the permeability barrier after fertilization in C. elegans are distinct in their timing and mechanism.

SUMMARY:How phospholipids are trafficked between the bacterial inner and outer membranes through the intervening hydrophilic space of the periplasm is not known. Here we report that members of the mammalian cell entry (MCE) protein family form structurally diverse hexameric rings and barrels with a central channel capable of mediating lipid transport. The E. coli MCE protein, MlaD, forms a ring as part of a larger ABC transporter complex in the inner membrane, and employs a soluble lipid-binding protein to ferry lipids between MlaD and an outer membrane protein complex. In contrast, EM structures of two other E. coli MCE proteins show that YebT forms an elongated tube consisting of seven stacked MCE rings, and PqiB adopts a syringe-like architecture. Both YebT and PqiB create channels of sufficient length to span the entire periplasmic space. This work reveals diverse architectures of highly conserved protein-based channels implicated in the transport of lipids between the inner and outer membranes of bacteria and some eukaryotic organelles.2 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/064360 doi: bioRxiv preprint first posted online Jul. 18, 2016; KEYWORDS: MCE, mammalian cell entry, lipids, transport, E. coli, periplasm, cryo-EM, crystallography, structure, outer membrane, bacteria 3 peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/064360 doi: bioRxiv preprint first posted online Jul. 18, 2016;

Just as the size of an organism is carefully controlled, the size of intracellular structures must also be regulated. The mitotic spindle is a supramolecular machine that generates the forces which separate sister chromatids during mitosis. Although spindles show little size variation between cells of the same type, spindle length can vary at least 10-fold between different species. Recent experiments on spindle length showed that in embryonic systems spindle length varied with blastomere size. Furthermore, a comparison between two Xenopus species showed that spindle length was dependent on some cytoplasmic factor. These data point toward mechanisms to scale spindle length with cell size. Centrosomes play an important role in organizing microtubules during spindle assembly. Here we use Caenorhabditis elegans to study the role of centrosomes in setting spindle length. We show that spindle length correlates with centrosome size through development and that a reduction of centrosome size by molecular perturbation reduces spindle length. By systematically analyzing centrosome proteins, we show that spindle length does not depend on microtubule density at centrosomes. Rather, our data suggest that centrosome size sets mitotic spindle length by controlling the length scale of a TPXL-1 gradient along spindle microtubules.