Calcium and the mechanism of axoplasmic transport

Using desheathed cat peroneal nerves in in vitro studies, Ca2+ was recently shown to be required to maintain axoplasmic transport. Calmodulin was also shown to be present in nerve and to participate in transport. These findings open up new possibilities for a better understanding of the underlying mechanism of transport. In the transport filament model, the materials transported are bound to a common carrier, the transport filaments, which are moved along the microtubules by means of an interaction with the side arms of the microtubules. This is an energy-requiring process that depends on a supply of ATP, which is utilized by the Ca2+,Mg2+-ATPase associated with the side arms of the microtubules. The Ca2+,Mg2+-ATPase is activated by calmodulin at the low micromolar levels of free Ca2+ present in the axon. The level is kept low by calcium-regulatory mechanisms that include mitochondria, endoplasmic reticulum, and calcium-binding proteins. Nerves exposed to higher-than-normal concentrations of Ca2+ in the medium show an increased number of particles in these organelles as expected of their Ca2+-regulatory role. The nature of the calmodulin-Ca,Mg-ATPase complex associated with the side arms is discussed on the basis of the transport model. Also discussed is slow transport, which is explained on the basis of the model as a differential binding affinity to the transport filaments.     

Quantitative aspects of the feeder cell phenomenon: mechanistic implications

It has been proposed that feeder cells function by supplying lymphocytes with the amino acid cysteine (a thiol compound). The results presented here indicate that thiols are the critical element of the feeder cell phenomenon. Specifically, we noted that the rank of thiol production by four different feeder cell lines corresponds to their relative abilities to support a lymphocyte cell line, CTLL-2. In addition, increasing thiol production by the feeder cells with lipopolysaccharide increased their support of CTLL-2 cells and decreasing it with homocysteate decreased support of CTLL-2 cells. However, it was also noted that substantial (up to 79% maximal) support of CTLL-2 growth was provided by feeder cell concentrations which could not produce detectable levels of free thiols. This prompted us to propose an alternative mechanism for the feeder effect which would explain these apparently paradoxical findings.     

Lipid-dependent recovery of alpha-bungarotoxin and monoclonal antibody binding to the purified alpha-subunit from Torpedo marmorata acetylcholine receptor. Enhancement by noncompetitive channel blockers

Recently the purified alpha-subunit from Torpedo marmorata acetylcholine receptor was shown to bind alpha-bungarotoxin with a KD approximately 3 nM in the presence of sodium dodecyl sulfate (Tzartos, S.J., and Changeux, J.P. (1983) EMBO J. 2, 381-387). Here we describe a further significant step toward renaturation of the alpha-subunit as judged by toxin and monoclonal antibody binding. Purified T. marmorata receptor subunits were diluted with 1% lipids (asolectin) plus 0.5% Na+ cholate. An anion-exchange resin eliminated most of the detergents, leaving approximately 0.1% Na+ cholate and the lipids. After this treatment, about 20% of the alpha-subunit recovered (but not the beta-, gamma-, or delta-subunit) exhibited a high affinity for radioiodinated alpha-bungarotoxin with a KD approximately 0.5 nM. The 34,000- and 27,000-dalton proteolytic peptides of the alpha-subunit conserved this lipid-dependent toxin binding. Unlabeled alpha-toxins, hexamethonium, and carbamylcholine competed with alpha-bungarotoxin for the renatured alpha-subunit. Noncompetitive channel blockers doubled the lipid-dependent toxin-binding capacity of the alpha-subunit but had no effect on the 27,000-dalton peptide. The binding of several monoclonal antibodies to the main immunogenic region (which is particularly sensitive to denaturation) significantly increased. In particular, binding of antibody 16 changed from 1% to denatured to 100% to the lipid-renaturated alpha-subunit. The binding of these antibodies was lost with the lipid-renatured 34,000- and 27,000-dalton peptides.     

The input of terrestrial invertebrates from tree canopies to a stream

SUMMARY. The input of terrestrial invertebrates from different tree canopies to a trout stream was determined for a 28-week period from April to October, 1980. Sycamore produced the greatest number of animals, followed by oak and alder. Ash was not significantly different from the controls. Coleoptera, Diptera, Homoptera and Arachnida made up the greatest number of animals caught, with Lepidoptera larvae important beneath oak. The input of biomass (g m^-2 dry wt) was also greatest beneath sycamore (35.80), followed by oak (27.76), alder (20.39), ash (11.15) and control (9.92). The input of biomass was bimodal. The significance of terrestrial invertebrates as food for salmonids is discussed.     

2-(2-Pyridyl)ethyl group: new type protecting group in the synthesis of DNA via phosphoramidite intermediates

2-(2-Pyridyl)ethyl group is a new type P-O protecting group for the synthesis of oligodeoxyribonucleotides by the phosphite triester method. This group is stable to alkali and acid conditions, and to be removed from internucleotidic bonds under mild conditions via two step procedures without any side reactions. Further we have found that bis(diisopropylamino)chlorophosphine is much more effective for the preparation of bis(diisopropylamino)alkoxyphosphines than various dichlorophosphines.     

Group-selective reagent modification of the sodium- and chloride-coupled glycine transporter under native and reconstituted conditions.

Glycine transporter from rat brain stem and spinal cord is inactivated by specific sulfhydryl reagents. Modification of lysine residues also promotes a decrease of the transporter activity but in a lesser extent than that promoted by thiol group reagents. Mercurials showed a more marked inhibitory effect than maleimide derivatives. SH groups display a similar reactivity for p-chloromercuribenzenesulfonate (pCMBS) and mersalyl in synaptosomal membrane vesicles and proteoliposomes reconstituted with the solubilized transporter. However, different reactivity is observed with N-ethylmaleimide (MalNEt), the greatest effect being attained in membrane vesicles. The rate of inactivation by pCMBS and MalNEt is pseudo-first-order showing time- and concentration-dependence. pCMBS and MalNEt decrease the Vmax for glycine transport and to a lesser extent act on the apparent Km. Treatment with dithiothreitol (DTT) of the transporter modified by pCMBS results in a complete restoration of transporter activity indicating that the effect exercised by the reagent is specific for cysteine residues on the protein. It is concluded that SH groups are involved in the glycine transporter function and that these critical residues are mostly located in a relatively hydrophilic environment of the protein.