Assessment of inner dynein arm structure and possible function in ciliary and flagellar axonemes

The construction and assessment of a three-dimensional computer-generated model of inner dynein arms on a 96-nm repeat unit of an axonemal doublet is described. The model is based on published electron micrographs of axonemes from Tetrahymena cilia and eel sperm, which were prepared using several different techniques: negative stain, freeze etch, and thin section. The inner arm structure is represented as three inner dynein arm complexes containing four inner dynein arms (IDAs), three dyads, and one single-headed arm, each capable of bridging the interdoublet gap. The IDA structures in the model have been correlated with the domains containing dynein heavy-chain isoforms mapped by several authors using genetic analyses of Chlamydomonas mutants. The model is consistent with micrographic evidence from axonemes of cilia and flagella from other organisms that led previously to conflicting structural interpretations. In this reconciling interpretation, the different alignments of the IDAs relative to the corresponding outer dynein arms observed in micrographs of differently prepared samples, result from the IDAs being arrested at different stages of their cycles of activity in each preparation. By interpolating between these positions of arrest, cycles of activity are proposed for each of the IDAs during which the arms attach to the neighbouring doublet microtubule and drive it tipwards.

Drosophila roadblock and Chlamydomonas LC7: a conserved family of dynein-associated proteins involved in axonal transport, flagellar motility, and mitosis

Eukaryotic organisms utilize microtubule-dependent motors of the kinesin and dynein superfamilies to generate intracellular movement. To identify new genes involved in the regulation of axonal transport in Drosophila melanogaster, we undertook a screen based upon the sluggish larval phenotype of known motor mutants. One of the mutants identified in this screen, roadblock (robl), exhibits diverse defects in intracellular transport including axonal transport and mitosis. These defects include intra-axonal accumulations of cargoes, severe axonal degeneration, and aberrant chromosome segregation. The gene identified by robl encodes a 97-amino acid polypeptide that is 57% identical (70% similar) to the 105-amino acid Chlamydomonas outer arm dynein-associated protein LC7, also reported here. Both robl and LC7 have homology to several other genes from fruit fly, nematode, and mammals, but not Saccharomyces cerevisiae. Furthermore, we demonstrate that members of this family of proteins are associated with both flagellar outer arm dynein and Drosophila and rat brain cytoplasmic dynein. We propose that roadblock/LC7 family members may modulate specific dynein functions.

An investigation of electron spin resonance in wild type Chlamydomonas reinhardi and mutant strains having impaired photosynthesis

The electron spin resonance signals of wild type Chlamydomonas reinhardi and three mutant strains having impaired photosynthesis have been investigated. The wild type strain generates two different electron spin resonance signals. Signal I is obtained without illumination (i.e., dark signal) whereas signal II is generated preferentially only by red light. Signal I is missing from wild type cells that have been cultured in the dark, but it returns after these dark-grown cells have been illuminated. Chloroplast fragments obtained from the three mutant strains cannot photoreduce TPN. Two of the strains lack the dark signal I while the third strain has both signal I and signal II. Other studies have revealed that the two mutant strains which lack signal I give no Hill reaction but that they can photoreduce TPN if supplied with an artificial reductant. The mutant strain which has both electron spin resonance signals can carry out the Hill reaction, yet it too will not photoreduce TPN unless reductant is supplied. The electron spin resonance signals generated by the wild type and mutant strains are discussed in terms of the pathway of TPN photoreduction, and it is suggested that signal I is associated with one of the two light-dependent phases of this pathway.

Ultrastructure of DNA-containing areas in the chloroplast of Chlamydomonas

The chloroplast of Chlamydomonas moewusii was examined by electron microscopic and cytochemical methods for the possible presence of DNA. Both the Feulgen reaction and acridine orange indicated the presence within the chloroplast of one or more irregularly shaped DNA-containing bodies generally in the vicinity of the pyrenoid. Electron micrographs revealed 25 A microfibrils in these areas which correspond to DNA macromolecules with respect to their location, morphology, and sensitivity to deoxyribonuclease digestion. The possibility that this material is the genetic system of the chloroplast and the hypothesis that the chloroplast represents an evolved endosymbiont are discussed.

Analysis of lipid components in zooplankter individuals by reactive pyrolysis-gas chromatography in the presence of organic alkali

It has been eagerly requested to develop a highly-sensitive method to characterize extremely minute amounts of natural organic materials occluded in meteorites and/or space dusts in order to confirm the existence of life in the extraterrestrial space. In this article, the reactive pyrolysis-gas chromatography (Py-GC) applied to the analysis of the lipid components contained in every zooplankter individual is introduced for the sake of its potential extension to the characterization of trace amounts of the extraterrestrial organic materials. Here, Py-GC was applied to 1) the discriminative analysis among zooplankter individuals cultured in different food concentrations, and 2) the correlation analysis between the distributions of fatty acid components in the lipids of zooplankter individuals and ingested algae phytoplankton.

Mechanisms of graviperception and response in unicellular systems

This introduction to a symposium considers established principles of mechanoreception and the physiology of excitable cells as a background of gravireception. From the perspective of experimental work in protists, current developments in the treatment and interpretation of graviresponses are addressed.

Red snow: is it safe to eat? A pilot study

The objective of this study was to determine if the ingestion of snow contaminated with Chlamydomonas nivalis causes diarrhea. The design was a single-blind crossover. Subjects were seven healthy volunteers, aged 24-56 who ingested 500 g of snow contaminated with C. nivalis. Outcome was measured by stool and diarrhea frequency. No differences in stool or diarrhea frequency were noted between treatment and placebo groups. The conclusion is that in this pilot study, there was no evidence supporting the concern that ingestion of "red snow" causes diarrhea.

Recombinant microtubule-associated protein 2c reduces the dynamic instability of individual microtubules

The effects of purified recombinant microtubule-associated protein 2c (rMAP2c) on the dynamic instability of microtubules were examined by direct observation of individual microtubules in vitro by video-enhanced differential interference contrast light microscopy. Microtubules were grown in the absence or presence of varying concentrations of rMAP2c and were analyzed to determine growth rates, shortening rates, and the frequencies of conversion between growing and shortening phases. We found rMAP2c to stabilize microtubules dramatically. The most notable effect is a reduction in both the frequency of catastrophes (transitions from growth to shortening) and the mean length of shortening events: no microtubule catastrophes were observed at concentrations of rMAP2c as low as 1.06 microM in a solution of 10 microM tubulin. Even at lower rMAP2c concentrations, there is a marked stabilizing effect. As the concentration of rMAP2c increases, average growth rates increase slightly, shortening rates decrease, and the frequency of rescues (transitions from shortening to growth) increases significantly. Together, these changes in parameters produce a population of extremely stable microtubules in the presence of rMAP2c. This stabilization is consistent with a structural role for MAP2c during early postnatal neural development.

Three-dimensional structure of the kinesin head-microtubule complex

Kinesin is a microtubule (MT)-associated 'motor' molecule fundamental to organelle transport. Recently, various kinesin superfamily members (KIFs) have also been identified and suggested as being responsible for the transport of specific organelles. Kinesin is a heterotetramer composed of two heavy chains and two light chains. The heavy chains form two globular heads, a rod and a fan-like tail completed by the light chains. The globular head, which is composed of approximately 340 amino-terminal residues of the heavy chain, includes both ATP-binding and MT-binding domains, and its recombinant protein also has these properties. To improve the understanding of the mechanism of force generation by an MT-based molecular motor, kinesin, we report here the three-dimensional structure of the complex of a recombinant kinesin head and MTs, as revealed by helical reconstruction from cryo-electron micrographs. A kinesin head is a globular teardrop-like structure binding to the ridge of one protofilament of MTs. We have determined the polarity of the structure of the complex of MTs and the kinesin head in relation to MT polarity.