A unique tubulin antiserum attenuates the rate of poleward chromosome movement in anaphase.

An antiserum against tubulin, NS20, was previously shown to specifically attenuate both fast axonal transport in vivo (Johnston, K. M. et al., Brain Res. 385, 38-45 (1986)) and in vitro (Johnston, K. M. et al., Cell Motil. Cytoskel. 7, 110-115 (1987)) and flagellar motility (Goldsmith, M. et al., Cell Motil. Cytoskel. 20, 249-262 (1991)). We hypothesized that NS20 blocked motility by binding to a multifunctional motor binding domain on the microtubules (MTs), or axonemes. Here we have examined the effect of microinjecting NS20, at metaphase, into dividing PtK2 cells. Plotting chromosome separation (CS) as a function of time, we report here that CS rates for anaphase A (chromosome-to-pole movement) were reduced by approximately 50% relative to uninjected controls. CS rates for anaphase B (spindle pole elongation) were unaffected by the NS20 antiserum. The inhibition of CS rate during anaphase A by NS20 was significantly greater than the inhibition caused by a control antitubulin serum (PC5). Two possible mechanisms underlying NS20's inhibition of CS during anaphase A were considered. NS20 could block the binding of a kinetochore-associated motor to kinetochore MTs (kMTs) or, alternatively, NS20 could stabilize kMTs against depolymerization. Our results favor the first alternative. In a cold-induced depolymerization assay, NS20 had no selective stabilizing effect on MTs. Moreover, we show that NS20 can selectively block the binding of a well characterized MT-associated motor (kinesin) to MTs, in vitro. These results suggest that NS20 may be defining a unique tubulin binding domain common to the motors underlying vesicle transport, flagellar motility, and chromosome movements during anaphase A.     

Analysis of data in changing temperature experiments

The use of a summative method of prredicting organism development under fluctuating environmental conditions based on fixed environment results is considered where time of entry to the environment is variable. Methods of comparing predicted with observed development under fluctuating environment are developed where starting and finishing times are available for each organism and relatable to each other and for the less informative case, where the times are available but not relatable to each other for each organism. Methods for calculating the variance of the comparison of predicted versus observed development for the two cases are presented.     

The effects of dimethyl sulfoxide on the osmotic properties of a rat megakaryocytopoietic cell line

Dimethyl sulfoxide (DMSO) at 0.6 M alters the osmotic state of intracellular water in a line of rat megakaryocytopoietic cells. The response is a function of temperature. Maximal structuring of water occurs at 24.7 degrees C, while disorganization occurs at 30 and 37 degrees C. Little effect occurs at 3 and at 18 degrees C. Parallel measurements of membrane permeability to water indicate that DMSO inhibits the osmotic exit of water at all temperatures. Maximal inhibition occurs at 24.7 degrees C, the same temperature at which maximal organization of water occurs. These findings emphasize the possible role which water may play in explaining the diverse functions of DMSO as cryogenic substance, promoter of cell fusion, and stimulator of cell differentiation.     

Linked and threaded loops in proteins

Tertiary structure of globular proteins has traditionally been analyzed in terms of the organization of secondary structure elements. This paper presents a method for systematically identifying different topological features of the convolutions of the backbone. We define a loop as a segment of chain whose end residues are in contact. We find some loops which are threaded by another segment of chain passing through the loop or actually linked with another loop. Fifty-six loop threadings were found among the 20 proteins studied, all of them occurring in a subset of seven proteins. In our sample, threadings and linkings were generally found if and only if the protein has more than 200 residues. To account for the existence of these topological features, despite their apparent entropic unfavorability, we have proposed a number of kinetic mechanisms by which they may form without a thread actually passing through a loop. We have found that almost all loop threadings possess structural features that would make one of these mechanisms plausible.