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FtsZ, the tubulin homolog essential for bacterial cell division, assembles the Z-ring at the division site, and directs peptidoglycan synthesis by its treadmilling action. A key unanswered question is how FtsZ achieves its kinetic polarity that drives treadmilling. To obtain insights into fundamental features of FtsZ assembly dynamics independent of peptidoglycan synthesis, we report the characterization of FtsZ from the cell wall-less bacteriumSpiroplasma melliferum(SmFtsZ). SmFtsZ was a slow GTPase and required higher critical concentration (CC) for polymerization as compared toEscherichia coliFtsZ (EcFtsZ). In FtsZs, a conformational switch from R- to T-state favors polymerization. In the crystal structures of SmFtsZ, conformation of the GTP-bound N-terminal domain (NTD) was in the T-state, while the relative orientation of the NTD and the C-terminal domain (CTD) matched the R-state. Hence, the crystal structures represent a conformational state that facilitates preferential binding of the NTD of the monomeric GTP-bound SmFtsZ to the CTD-exposed end of SmFtsZ filament. CTD of the nucleotide-bound monomer cannot bind to the NTD-exposed end of the filament unless relative rotation of the domains leads to cleft opening. The mutation F224M in SmFtsZ cleft resulted in higher GTPase activity and lower CC, whereas the corresponding M225F in EcFtsZ resulted in cell division defects inE. coli. These results suggest that relative rotation of the domains leading to cleft opening is a rate-limiting step of polymerization. This slows the addition of monomers to the NTD-exposed end of the filament in comparison to the CTD end, thus explaining kinetic polarity.

Muscular dystrophies are a clinically and heterogeneous group of disorders that all share clinical characteristics of progressive muscular weakness. Duchenne muscular dystrophy (DMD) is the most common X-linked disorder muscular dystrophy in children, presenting in early childhood and characterized by proximal muscle weakness and calf hypertrophy in affected boys. There is usually delay in motor development and eventually wheelchair confinement followed by premature death from cardiac or respiratory complications. Treatment modalities such as corticosteroid therapy and use of intermittent positive pressure ventilation have provided improvements in function, ambulation, quality of life, and life expectancy, although novel therapies still aim to provide a cure for this devastating disorder. Here, we present a case of DMD in a 12-year-old male with remarkable clinical and oral manifestations.

ComplexinII and SynaptotagminI coordinately transform the constitutively active SNARE-mediated fusion mechanism into a highly synchronized, Ca2+-triggered release apparatus., ComplexinII (CpxII) and SynaptotagminI (SytI) have been implicated in regulating the function of SNARE proteins in exocytosis, but their precise mode of action and potential interplay have remained unknown. In this paper, we show that CpxII increases Ca2+-triggered vesicle exocytosis and accelerates its secretory rates, providing two independent, but synergistic, functions to enhance synchronous secretion. Specifically, we demonstrate that the C-terminal domain of CpxII increases the pool of primed vesicles by hindering premature exocytosis at submicromolar Ca2+ concentrations, whereas the N-terminal domain shortens the secretory delay and accelerates the kinetics of Ca2+-triggered exocytosis by increasing the Ca2+ affinity of synchronous secretion. With its C terminus, CpxII attenuates fluctuations of the early fusion pore and slows its expansion but is functionally antagonized by SytI, enabling rapid transmitter discharge from single vesicles. Thus, our results illustrate how key features of CpxII, SytI, and their interplay transform the constitutively active SNARE-mediated fusion mechanism into a highly synchronized, Ca2+-triggered release apparatus.