During mitosis, adherent animal cells undergo a drastic shape change, from essentially flat to round. Mitotic cell rounding is thought to facilitate organization within the mitotic cell and be necessary for the geometric requirements of division. However, the forces that drive this shape change remain poorly understood in the presence of external impediments, such as a tissue environment. Here we use cantilevers to track cell rounding force and volume. We show that cells have an outward rounding force, which increases as cells enter mitosis. We find that this mitotic rounding force depends both on the actomyosin cytoskeleton and the cells' ability to regulate osmolarity. The rounding force itself is generated by an osmotic pressure. However, the actomyosin cortex is required to maintain this rounding force against external impediments. Instantaneous disruption of the actomyosin cortex leads to volume increase, and stimulation of actomyosin contraction leads to volume decrease. These results show that in cells, osmotic pressure is balanced by inwardly directed actomyosin cortex contraction. Thus, by locally modulating actomyosin-cortex-dependent surface tension and globally regulating osmotic pressure, cells can control their volume, shape and mechanical properties.

The microtubule cytoskeleton and the mitotic spindle are highly dynamic structures, yet their sizes are remarkably constant, thus indicating that the growth and shrinkage of their constituent microtubules are finely balanced. This balance is achieved, in part, through kinesin-8 proteins (such as Kip3p in budding yeast and KLP67A in Drosophila) that destabilize microtubules. Here, we directly demonstrate that Kip3p destabilizes microtubules by depolymerizing them--accounting for the effects of kinesin-8 perturbations on microtubule and spindle length observed in fungi and metazoan cells. Furthermore, using single-molecule microscopy assays, we show that Kip3p has several properties that distinguish it from other depolymerizing kinesins, such as the kinesin-13 MCAK. First, Kip3p disassembles microtubules exclusively at the plus end and second, remarkably, Kip3p depolymerizes longer microtubules faster than shorter ones. These properties are consequences of Kip3p being a highly processive, plus-end-directed motor, both in vitro and in vivo. Length-dependent depolymerization provides a new mechanism for controlling the lengths of subcellular structures.

The microtubule cytoskeleton is a dynamic structure in which the lengths of the microtubules are tightly regulated. One regulatory mechanism is the depolymerization of microtubules by motor proteins in the kinesin-13 family 1 . These proteins are crucial for the control of microtubule length in cell division [2][3][4] , neuronal development 5 and interphase microtubule dynamics 6,7 . The mechanism by which kinesin-13 proteins depolymerize microtubules is poorly understood. A central question is how these proteins target to microtubule ends at rates exceeding those of standard enzyme-substrate kinetics 8 . To address this question we developed a single-molecule microscopy assay for MCAK, the founding member of the kinesin-13 family 9 . Here we show that MCAK moves along the microtubule lattice in a one-dimensional (1D) random walk. MCAK-microtubule interactions were transient: the average MCAK molecule diffused for 0.83 s with a diffusion coefficient of 0.38 mm 2 s 21 . Although the catalytic depolymerization by MCAK requires the hydrolysis of ATP, we found that the diffusion did not. The transient transition from three-dimensional diffusion to 1D diffusion corresponds to a "reduction in dimensionality" 10 that has been proposed as the search strategy by which DNA enzymes find specific binding sites 11 . We show that MCAK uses this strategy to target to both microtubule ends more rapidly than direct binding from solution.Kinesin-13 motor proteins act at microtubule ends, where they are thought to force protofilaments into a curved conformation 12,13 , which is a likely structural intermediate in the depolymerization process 14 . Classically, kinesin motor proteins reach microtubule ends by ATP-dependent translocation along microtubules. However, LETTERSFigure 1 | MCAK-dependent microtubule depolymerization. a, Diagram of the in vitro assay depicting a microtubule (red) immobilized above the glass surface by anti-tubulin antibodies (dark blue). Excitation by total internal reflection allows the detection of single molecules (namely MCAK-GFP in green) in the evanescent field (shown in blue). b, Epifluorescence images of immobilized microtubules at the times shown in minutes. MCAK dimer (8 nM) was added at t ¼ 2 min. c, Plot of microtubule depolymerization rate against MCAK concentration. Error bars are s.d. Data fitted to Hill equations (lines plotted) yielded K m ¼ 3.9 nM and K m ¼ 6.1 nM for MCAK and MCAK-GFP, and n ¼ 2.4 and n ¼ 2.2, respectively. Red squares, MCAK-His 6 ; green circles, MCAK-His 6 -EGFP. d, Shortening of four microtubules from a mean length of 8.4 mm to 7.8 mm (black line) after the addition of 5 nM MCAK (red line). The depolymerization rate approached steady state with a time constant of 3.8 s (green fitted line). (Fig. 1a) in which microtubules were immobilized on coverslips by means of surface-adsorbed anti-tubulin antibodies. Individual rhodamine-labelled microtubules and single MCAK-GFP molecules were revealed by epifluorescence and total-internalreflection fluorescence (TIRF) illumina...

The controlled adhesion of cells to each other and to the extracellular matrix is crucial for tissue development and maintenance. Numerous assays have been developed to quantify cell adhesion. Among these, the use of atomic force microscopy (AFM) for single-cell force spectroscopy (SCFS) has recently been established. This assay permits the adhesion of living cells to be studied in near-physiological conditions. This implementation of AFM allows unrivaled spatial and temporal control of cells, as well as highly quantitative force actuation and force measurement that is sufficiently sensitive to characterize the interaction of single molecules. Therefore, not only overall cell adhesion but also the properties of single adhesion-receptor–ligand interactions can be studied. Here we describe current implementations and applications of SCFS, as well as potential pitfalls, and outline how developments will provide insight into the forces, energetics and kinetics of cell-adhesion processes.

Biological processes rely on molecular interactions that can be directly measured using force spectroscopy techniques. Here we review how atomic force microscopy can be applied to force probe surfaces of living cells to single-molecule resolution. Such probing of individual interactions can be used to map cell surface receptors, and to assay the receptors' functional states, binding kinetics and landscapes. This information provides unique insight into how cells structurally and functionally modulate the molecules of their surfaces to interact with the cellular environment.

Addition of the reducing agent dithiothreitol (DTT) to the medium of living cells prevented disulfide bond formation in newly synthesized influenza hemagglutinin (HA0) and induced the reduction of already oxidized HA0 inside the ER. The reduced HA0 did not trimerize or leave the ER. When DTT was washed out, HA0 was rapidly oxidized, correctly folded, trimerized and transported to the Golgi complex. We concluded that protein folding and the redox conditions in the ER can be readily manipulated by addition of DTT without affecting most other cellular functions, that the reduced influenza HA0 remains largely unfolded, and that folding events that normally take place on the nascent HA0 chains can be delayed and induced post‐translationally without loss in efficiency.

No abstract

During mitosis, adherent cells round up, by increasing the tension of the contractile actomyosin cortex while increasing the internal hydrostatic pressure. In the simple scenario of a liquid cell interior, the surface tension is related to the local curvature and the hydrostatic pressure difference by Laplace's law. However, verification of this scenario for cells requires accurate measurements of cell shape. Here, we use wedged micro-cantilevers to uniaxially confine single cells and determine confinement forces while concurrently determining cell shape using confocal microscopy. We fit experimentally measured confined cell shapes to shapes obeying Laplace's law with uniform surface tension and find quantitative agreement. Geometrical parameters derived from fitting the cell shape, and the measured force were used to calculate hydrostatic pressure excess and surface tension of cells. We find that HeLa cells increase their internal hydrostatic pressure excess and surface tension from ≈ 40 Pa and 0.2 mNm−1 during interphase to ≈ 400 Pa and 1.6 mNm−1 during metaphase. The method introduced provides a means to determine internal pressure excess and surface tension of rounded cells accurately and with minimal cellular perturbation, and should be applicable to characterize the mechanical properties of various cellular systems.