An integrated theoretical model reveals how the chemical and the mechanical aspects of endocytosis are coordinated coherently in yeast cells, driving progression through the endocytic pathway and ensuring efficient vesicle scission in vivo.

Endocytosis in budding yeast is thought to occur in several phases. First, the membrane invaginates and then elongates into a tube. A vesicle forms at the end of the tube, eventually pinching off to form a ''free'' vesicle. Experiments show that actin polymerization is an active participant in the endocytic process, along with a number of membrane-associated proteins. Here we investigate the possible roles of these components in driving vesiculation by constructing a quantitative model of the process beginning at the stage where the membrane invagination has elongated into a tube encased in a sheath of membrane-associated protein. This protein sheath brings about the scission step where the vesicle separates from the tube. When the protein sheath is dynamin, it is commonly assumed that scission is brought about by the constriction of the sheath. Here, we show that an alternative scenario can work as well: The protein sheath acts as a ''filter'' to effect a phase separation of lipid species. The resulting line tension tends to minimize the interface between the tube region and the vesicle region. Interestingly, large vesicle size can further facilitate the reduction of the interfacial diameter down to a few nanometers, small enough so that thermal fluctuations can fuse the membrane and pinch off the vesicle. To deform the membrane into the tubular vesicle shape, the membrane elastic resistance forces must be balanced by some additional forces that we show can be generated by actin polymerization and͞or myosin I. These active forces are shown to be important in successful scission processes as well.actin ͉ dynamin ͉ endocytosis ͉ vesiculation ͉ mathematical model

Bacterial plasmids are extrachromosomal DNA that provides selective advantages for bacterial survival. Plasmid partitioning can be remarkably robust. For high-copy-number plasmids, diffusion ensures that both daughter cells inherit plasmids after cell division. In contrast, most low-copy-number plasmids need to be actively partitioned by a conserved tripartite ParA-type system. ParA is an ATPase that binds to chromosomal DNA; ParB is the stimulator of the ParA ATPase and specifically binds to the plasmid at a centromere-like site, parS. ParB stimulation of the ParA ATPase releases ParA from the bacterial chromosome, after which it takes a long time to reset its DNA-binding affinity. We previously demonstrated in vitro that the ParA system can exploit this biochemical asymmetry for directed cargo transport. Multiple ParA-ParB bonds can bridge a parS-coated cargo to a DNA carpet, and they can work collectively as a Brownian ratchet that directs persistent cargo movement with a ParA-depletion zone trailing behind. By extending this model, we suggest that a similar Brownian ratchet mechanism recapitulates the full range of actively segregated plasmid motilities observed in vivo. We demonstrate that plasmid motility is tuned as the replenishment rate of the ParA-depletion zone progressively increases relative to the cargo speed, evolving from diffusion to pole-to-pole oscillation, local excursions, and, finally, immobility. When the plasmid replicates, the daughters largely display motilities similar to that of their mother, except that when the single-focus progenitor is locally excursive, the daughter foci undergo directed segregation. We show that directed segregation maximizes the fidelity of plasmid partition. Given that local excursion and directed segregation are the most commonly observed modes of plasmid motility in vivo, we suggest that the operation of the ParA-type partition system has been shaped by evolution for high fidelity of plasmid segregation.

During endocytic vesicle formation, distinct subdomains along the membrane invagination are specified by different proteins, which bend the membrane and drive scission. Bin-Amphiphysin-Rvs (BAR) and Fer-CIP4 homology-BAR (F-BAR) proteins can induce membrane curvature and have been suggested to facilitate membrane invagination and scission. Two F-BAR proteins, Syp1 and Bzz1, are found at budding yeast endocytic sites. Syp1 arrives early but departs from the endocytic site before formation of deep membrane invaginations and scission. Using genetic, spatiotemporal, and ultrastructural analyses, we demonstrate that Bzz1, the heterodimeric BAR domain protein Rvs161/167, actin polymerization, and the lipid phosphatase Sjl2 cooperate, each through a distinct mechanism, to induce membrane scission in yeast. Additionally, actin assembly and Rvs161/167 cooperate to drive formation of deep invaginations. Finally, we find that Bzz1, acting at the invagination base, stabilizes endocytic sites and functions with Rvs161/167, localized along the tubule, to achieve proper endocytic membrane geometry necessary for efficient scission. Together, our results reveal that dynamic interplay between a lipid phosphatase, actin assembly, and membrane-sculpting proteins leads to proper membrane shaping, tubule stabilization, and scission.