Synthetic peptides corresponding to the calmodulin-binding domains of skeletal muscle myosin light chain kinase and human erythrocyte Ca2+ pump interact with and permeabilize liposomes and cell membranes

Rapid entry of bitter and sweet tastants into liposomes and taste cells: implications for signal transduction

Some amphipathic bitter tastants and non-sugar sweeteners are direct activators of G proteins and stimulate transduction pathways in cells not related to taste. We demonstrate that the amphipathic bitter tastants quinine and cyclo(Leu-Trp) and the non-sugar sweetener saccharin translocate rapidly through multilamellar liposomes. Furthermore, when rat circumvallate (CV) taste buds were incubated with the above tastants for 30 s, their intracellular concentrations increased by 3.5- to 7-fold relative to their extracellular concentrations. The time course of this dramatic accumulation was also monitored in situ in rat single CV taste buds under a confocal laser-scanning microscope. Tastants were clearly localized to the taste cell cytosol. It is proposed that, due to their rapid permeation into taste cells, these amphipathic tastants may be available for activation of signal transduction components (e. g., G proteins) directly within the time course of taste sensation. Such activation may occur in addition to the action of these tastants on putative G protein-coupled receptors. This phenomenon may be related to the slow taste onset and lingering aftertaste typically produced by many bitter tastants and non-sugar sweeteners.

Heme as an optical probe for studying the interactions between calmodulin and the Ca(2+)-ATPase of the human erythrocyte membrane

The heme group was used as an optical probe to study the interactions between calmodulin and its targets: the peptide melittin and the enzyme Ca(2+)-ATPase. As already reported, melittin when present in Tris buffer binds hemin-CN which quenches the tryptophan fluorescence. Addition of calmodulin restores the fluorescence significantly accompanied by a blue shift. We show here that the recovery of fluorescence is very slow and takes about 120 min to become constant. In a hydrophobic buffer, the fluorescence spectrum of melittin is already shifted with a peak at 335 nm and intensity almost 2-fold relative to a similar concentration of melittin in Tris buffer. The quenching of tryptophan fluorescence is lesser in this buffer and further addition of calmodulin fails to restore the fluorescence. This indicates the absence of binding of calmodulin to melittin in hydrophobic conditions. Under similar conditions of hydrophobicity, hemin-CN quenches about 35% of the tryptophan fluorescence of the Ca(2+)-ATPase. The subsequent addition of calmodulin restores about half of the quenched fluorescence. The interaction of calmodulin with the Ca(2+)-ATPase even under hydrophobic conditions suggests its high specificity for the enzyme which may be expected for a physiological target.

Rat myr 4 defines a novel subclass of myosin I: identification, distribution, localization, and mapping of calmodulin-binding sites with differential calcium sensitivity

We report the identification and characterization of myr 4 (myosin from rat), the first mammalian myosin I that is not closely related to brush border myosin I. Myr 4 contains a myosin head (motor) domain, a regulatory domain with light chain binding sites and a tail domain. Sequence analysis of myosin I head (motor) domains suggested that myr 4 defines a novel subclass of myosin I's. This subclass is clearly different from the vertebrate brush border myosin I subclass (which includes myr 1) and the myosin I subclass(es) identified from Acanthamoeba castellanii and Dictyostelium discoideum. In accordance with this notion, a detailed sequence analysis of all myosin I tail domains revealed that the myr 4 tail is unique, except for a newly identified myosin I tail homology motif detected in all myosin I tail sequences. The Ca(2+)-binding protein calmodulin was demonstrated to be associated with myr 4. Calmodulin binding activity of myr 4 was mapped by gel overlay assays to the two consecutive light chain binding motifs (IQ motifs) present in the regulatory domain. These two binding sites differed in their Ca2+ requirements for optimal calmodulin binding. The NH2-terminal IQ motif bound calmodulin in the absence of free Ca2+, whereas the COOH-terminal IQ motif bound calmodulin in the presence of free Ca2+. A further Ca(2+)-dependent calmodulin binding site was mapped to amino acids 776-874 in the myr 4 tail domain. These results demonstrate a differential Ca2+ sensitivity for calmodulin binding by IQ motifs, and they suggest that myr 4 activity might be regulated by Ca2+/calmodulin. Myr 4 was demonstrated to be expressed in many cell lines and rat tissues with the highest level of expression in adult brain tissue. Its expression was developmentally regulated during rat brain ontogeny, rising 2-3 wk postnatally, and being maximal in adult brain. Immunofluorescence localization demonstrated that myr 4 is expressed in subpopulations of neurons. In these neurons, prominent punctate staining was detected in cell bodies and apical dendrites. A punctate staining that did not obviously colocalize with the bulk of F-actin was also observed in C6 rat glioma cells. The observed punctate staining for myr 4 is reminiscent of a membranous localization.