Two subtypes of sodium channel with tetrodotoxin sensitivity and insensitivity detected in denervated mammalian skeletal muscle.

The action potential (AP) in most nerve and muscle preparations depends upon nanomolar concentrations of the neurotoxins saxitoxin (STX) and tetrodotoxin (TTX). In some excitable tissues lacking mature innervation, a toxin-resistant AP has been described by electrophysiological results. However, multiple attempts to detect corresponding toxin-resistant Na channels with radiolabelled STX and TTX have been unsuccessful.

Two mutations in the dispensable part of alanine tRNA synthetase which affect the catalytic activity.

Two previously described chromosomal mutant alleles, alaS4 and alaS5, of Escherichia coli Ala-tRNA synthetase have been analyzed. Each causes a sharp diminution in aminoacylation activity and disrupts the alpha 4 tetramer structure of identical chains of 875 amino acids; neither mutation significantly disturbs the activity for synthesis of alanyladenylate. The location of each mutation within the structural gene has been mapped by marker rescue with specific gene fragments.

Size polymorphism and the structure of aminoacyl-tRNA synthetases.

Although aminoacyl-tRNA synthetases catalyze the same chemical reaction, the individual enzymes have a wide range of sizes. Proteolytic digestion has yielded active catalytic fragments of two synthetases. A set of gene deletions in a large synthetase has been used successfully in the creation of a variety of enzyme fragments that have been studied individually; a fragment with about half of the total polypeptide is sufficient to aminoacylate tRNA in vivo.

Dispensable pieces of an aminoacyl tRNA synthetase which activate the catalytic site.

Recent data suggest that size polymorphism of aminoacyl tRNA synthetase is due to variable fusions of additional functional domains to a catalytic core so that, in a large synthetase, a substantial part of the polypeptide is dispensable for catalytic activity. We demonstrate here that a dispensable domain, joined to the catalytic core of a large synthetase, can activate the catalytic sites. This is shown by complementation of an activity-deficient mutant enzyme by protein fragments that contain internal deletions within the catalytic domain and are themselves devoid of activity.

Modular arrangement of functional domains along the sequence of an aminoacyl tRNA synthetase.

Gene deletions show that much of Escherichia coli alanine tRNA synthetase is dispensable for each of three activities and that these activities appear to require specific domains arranged linearly along the polypeptide. Thus, variable fusions of extra polypeptide domains to a catalytic core may account for the diverse of aminoacyl tRNA synthetases.

Identification of two sodium channel subtypes in chick heart and brain.

Na+ channels in chick brain and heart have been directly compared by measuring binding of tritium-labeled saxitoxin ([3H]STX) to the two tissues under identical conditions. Maximum saturable uptake and toxin affinity were considerably less in chick heart than in chick brain, requiring the development of an assay method to resolve specific [3H]STX uptake in heart. With this method, binding to both preparations consisted of a specific saturable component and a linear nonspecific component.