The influence of doubly attached crossbridges on the mechanical behavior of skeletal muscle fibers under equilibrium conditions

Sequence- and interactome-based prediction of viral protein hotspots targeting host proteins: a case study for HIV Nef

Virus proteins alter protein pathways of the host toward the synthesis of viral particles by breaking and making edges via binding to host proteins. In this study, we developed a computational approach to predict viral sequence hotspots for binding to host proteins based on sequences of viral and host proteins and literature-curated virus-host protein interactome data. We use a motif discovery algorithm repeatedly on collections of sequences of viral proteins and immediate binding partners of their host targets and choose only those motifs that are conserved on viral sequences and highly statistically enriched among binding partners of virus protein targeted host proteins. Our results match experimental data on binding sites of Nef to host proteins such as MAPK1, VAV1, LCK, HCK, HLA-A, CD4, FYN, and GNB2L1 with high statistical significance but is a poor predictor of Nef binding sites on highly flexible, hoop-like regions. Predicted hotspots recapture CD8 cell epitopes of HIV Nef highlighting their importance in modulating virus-host interactions. Host proteins potentially targeted or outcompeted by Nef appear crowding the T cell receptor, natural killer cell mediated cytotoxicity, and neurotrophin signaling pathways. Scanning of HIV Nef motifs on multiple alignments of hepatitis C protein NS5A produces results consistent with literature, indicating the potential value of the hotspot discovery in advancing our understanding of virus-host crosstalk.

Strain dependence of the elastic properties of force-producing cross-bridges in rigor skeletal muscle

Stretch and release experiments carried out on skinned single fibers of frog skeletal muscle under rigor conditions indicate that the elastic properties of the fiber depend on strain. For modulation frequencies below 1000 Hz, the results show an increase in Young's modulus of 20% upon a stretch of 1 nm/half-sarcomere. Remarkably, the strain dependence of Young's modulus decreases at higher frequencies to about 10% upon a 1-nm/half-sarcomere stretch at a modulation frequency of 10 kHz. This suggests that the cause of the effect is less straightforward than originally believed: a simple slackening of the filaments would result in an equally large strain dependence at all frequencies, whereas strain-dependent properties of the actin filaments should show up most clearly at higher frequencies. We believe that the reduction of the strain dependence points to transitions of the cross-bridges between distinct force-producing states. This is consistent with the earlier observation that Young's modulus in rigor increases toward higher frequencies.

Equilibrium muscle cross-bridge behavior. Theoretical considerations. II. Model describing the behavior of strongly-binding cross-bridges when both heads of myosin bind to the actin filament

A model has been developed for characterizing the interaction between strongly-binding myosin cross-bridges and actin in muscle fibers under equilibrium conditions where both heads of the myosin cross-bridge bind to actin. The model, that of Anderson and Schoenberg (1987. Biophys. J. 52:1077-1082) is quite similar to that of Schoenberg (1985. Biophys. J. 48:467-475), except that explicit account is taken of the fact that each crossbridge has two heads which can bind to actin. The key assumption that allows this model to explain a large body of data unexplained by the Schoenberg (1985) model is that the two crossbridge heads are not totally independent of one another after attachment. After the first head attaches, the second head is then free to attach only to an actin site distal to the first head. This means that when the more distally attached head subsequently detaches and reattaches (as the heads continually do), it will not reattach in a position of lesser strain and reduce the force it supports, but instead will remain attached in its strained position until the proximally attached head also detaches. This model gives an explanation for two important and otherwise unexplained observations made previously: it explains why at ionic strengths in the range of 50-120 mM, (a) the rate constant of force decay after a small stretch is a sigmoidal function of nucleotide analogue concentration, and (b) why in the presence of analogues or in rigor the rate constant of force decay after a small stretch is significantly slower than the rate constant for myosin subfragment-1 detachment from actin in solution.

Cell-cell conjugation. Transient analysis and experimental implications

In the present study we investigate the transient conjugation of cell pairs by using a mathematical model. Macromolecules responsible for adhesion (bonds) are assumed to exist in two reversible states, attached and unattached, and exert a force elastic in nature only when they cross-link the two cell surfaces (attached state). Bonds form a link between the two cell surfaces only in the attached form. The unattached bridges are assumed laterally mobile in the plane of the cell membrane. Lateral mobility of attached bonds may be limited by structures on the undersurface of the cell membrane. Using this model we show that the bond density distribution between a cytotoxic T-cell (F-1) and a cancer cell (JY:HLA-A2-B7-DR4, W6) approaches equilibrium within 10 min, the incubation period used in experiments by Sung, K.L.P., L.A. Sung, M. Crimmins, S.J. Burakoff, and S. Chien (1986. Science [Wash. DC]. 234:1405-1408). If the diffusion coefficient of attached bonds is set equal to zero in the computations the model predictions indicate accumulation of bonds at the edge of conjugation. This prediction is consistent with present experimental data on lectin-induced red blood cell aggregation (Vayo, M., R. Skalak, P. Brunn, S. Usami, and S. Chien. 1987. Fed. Proc. 46:1043). It is concluded that significant features of micromanipulation data on specific adhesion can be explained by the diffusivity properties of bonds responsible for adhesion.

Slip of rabbit striated muscle in rigor or AMPPNP

Single glycerol-extracted rabbit psoas muscle fibres have been slowly extended either in rigor or in the unhydrolysable ATP analogue, AMPPNP, and their sarcomere length, sarcomere structure and tension measured. The length of regularly arrayed sarcomeres, measured by optical diffraction, increased continuously as the muscle was stretched; the maximum sarcomere extension seen was approximately 6%. In the electron microscope sarcomeres from extended muscle fixed in rigor or AMPPNP remained regular in their internal structure, without rupture or obvious lengthening around the Z line. During steady extension at 0.024% per min the tension in the muscle fibre rose until it reached a limiting value [Tm] when the sarcomeres had stretched by 0.8-1.6% and then remained constant with continued extension, while the sarcomeres continued to stretch. Provided that a novel form of preparation of the glycerol-extracted fibres was employed, Tm in rigor was a large fraction of the tension expected from an active isometric muscle fibre. In the presence of AMPPNP Tm was reduced by a factor of 2 to 3. Step extension by 0.08% at 5-min intervals gave the same pattern of mechanical response with similar values of Tm. The isometric tension decay in the interval between the steps was very rapid at first and slowed continuously until the next step. The average speed of tension fall between 30 and 300 s after stretch was measured at each step and plotted relative to the tension in the muscle. The relationship approached linearity, although with a significant upward curvature at high tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Adhesion induced by mobile cross-bridges: steady state peeling of conjugated cell pairs

Recent experiments on cell-cell adhesion indicate that (i) bonds between two cells migrate towards the region of conjugation during peeling, and (ii) cell membrane tension increases with the rate of peeling. Analytical methods that would allow the derivation of intrinsic parameters of adhesion from such experimental data have yet to be explored. In this study we introduce a specific adhesion model to investigate the complex interaction between rate of peeling, lateral diffusivity and the rate of detachment of bonds. Our analysis show that adhesive energy density increases with the rate of peeling and with lateral diffusivity of the bonds but decreases with increasing rate of detachment. The rate dependency of the adhesive energy is due to the modulation of the number density distribution of bonds by the speed of peeling.

Effect of adenosine triphosphate analogues on skeletal muscle fibers in rigor

It is commonly believed, for both vertebrate striated and insect flight muscle, that when the ATP analogue adenyl-5'-yl imidodiphosphate (AMPPNP) is added to the muscle fiber in rigor, it causes the fiber to lengthen by 0.15%. This has been interpretated (Marston S.B., C.D. Roger, and R.T. Tregear. 1976. J. Mol. Biol. 104:263-267) as suggesting (a) that in rigor the crossbridge is fixed to, i.e., almost never detaches from the actin filament; (b), that the crossbridge remains fixed to the actin filament after AMPPNP addition; and (c) that the ability of AMPPNP to cause apparent lengthening of a muscle fiber is due to its ability to cause a conformational change in the myosin crossbridge that has an axial component of approximately 1.6 nm/half-sarcomere. The present study, done only on chemically-skinned rabbit psoas fibers, confirms that AMPPNP can cause muscle fibers to lengthen by 0.15% but only for a narrow set of experimental conditions. When experimental conditions are varied over a wider range, it becomes apparent that the extent of lengthening of a rigor muscle fiber upon AMPPNP addition depends almost entirely on the strain present in the rigor fiber before AMPPNP addition. Addition of AMPPNP to an unstrained rigor fiber (one supporting zero tension), induces zero length change while addition of AMPPNP to very highly strained rigor fibers induces length changes greater than 0.15%. The data thus do not support the hypotheses that the crossbridges remain fixed to the actin filament after AMPPNP addition and that the size of the apparent length change induced by AMPPNP is related to the size of the axial component of a conformational change. Instead, the data support the idea that the ability of AMPPNP to cause lengthening of a rigor muscle fiber is related to its ability to accelerate the rate at which strained crossbridges detach from actin and reattach in positions in lesser strain. The data do not rule out a conformational change upon AMPPNP binding, they simply make clear that any attempt to measure a force response conceivably due to a conformational change, would be more than obscured by the force changes due to crossbridges detaching and reattaching in positions of lesser strain.