Firefly luciferase as a tool in molecular and cell biology

The unique properties of firefly luciferase and the cloning of the gene for this enzyme have spawned a number of novel applications of this protein. We summarize a few of these applications including its use as a reporter gene, as a model for the study of protein import into peroxisomes, and as a component of a heterologous gene expression system.     

Cell-cell recognition of host surfaces by pathogens. The adsorption of maize (Zea mays) root mucilage by surfaces of pathogenic fungi.

The adsorption of radioactive mucilage by pathogenic fungi was shown to be dependent upon time, the composition of mucilage, the type of fungal surface (conidia, hyphae, hyphal apices), fungal species, pH and bivalent cations. All fungal adhesins were inactivated by either proteinase or polysaccharase treatments. Adsorption was not inhibited by the numberous mono-, di- and oligo-saccharides that were tested individually, but it was inhibited absolutely by several polysaccharides. This suggested that adsorption of mucilage by pathogens involved conformational and ionic interactions between plant and fungal polymers but not fungal lectins bound to sugar residues of mucilage. Several fractionation schemes showed that pathogens bound only the most acidic of the variety of polymers that comprise mucilage. There was not any absolute distinction between ability to bind radioactive mucilage and type of pathogen or non-pathogen. However, there were notable differences in characteristics of adsorption between two types of pathogen. Differences were revealed by comparison of the adsorption capacities of conidia and germinant conidia and chromatography of radioactive mucilage on germinant conidia. An ectotrophic root-infecting fungus (a highly specialized pathogen) bound a greater proportion of mucilage than did a vascular-wilt fungus (of catholic host and tissue range) with more than one class of site for adsorption. In contrast with the vascular-wilt fungus, sites for adsorption on the specialized pathogen were present solely on surfaces formed by germination.     

A simple sensitive radioreceptor assay for calcium antagonist drugs

A radioreceptor assay for calcium channel antagonist drugs described here is based on the ability of these drugs to affect ³H-nitrendipine binding to calcium channels. All the known calcium channel antagonists may be assayed in this manner. The assay can detect 10–100 nM (4 – 40 ng/ml) nimodipine, 10–100 nM (3.5 – 35 ng/ml) nifedipine, 3–30 μM (1.2 – 12 μm/ml) prenylamine, 0.1 – 1.0 μM (49 – 490 ng/ml) verapamil and 3–30 μM (1.2 – 12 μg/ml) diltiazem. These values cover the range of concentrations of calcium channel antagonists that are clinically important. As the radioreceptor assay detects active metabolites as well as the parent drugs, it should prove a useful adjunct in cardiovascular therapy. The method is more reproducible, simpler and less expensive than other methods such as high pressure liquid chromatography.     

Schizosaccharomyces pombe skp1+ encodes a protein kinase related to mammalian glycogen synthase kinase 3 and complements a cdc14 cytokinesis mutant.

We report the cloning of the skp1+ gene, a Schizosaccharomyces pombe homolog of the glycogen synthase kinase 3 (GSK-3) family whose members in higher eukaryotes are involved in cell fate determination, nuclear signalling, and hormonal regulation. skp1 is 67% identical to mammalian GSK-3 beta and displays similar biochemical properties in vitro. Like GSK-3 beta, skp1 is phosphorylated on a conserved tyrosine residue, and this phosphorylation is required for efficient activity. skp1 is also phosphorylated at a serine which has been identified as S-335. Phosphorylation at this site is likely to inhibit its function. Unlike the mammalian enzyme, skp1 both tyrosine autophosphorylates in yeast cells and can phosphorylate other proteins on tyrosine in bacteria. The skp1+ gene is not essential. However, cells with deletions in skp1+ are sensitive to heat shock and exhibit defects in sporulation. Overexpression of wild-type skp1+ specifically complements cdc14-118, one of several mutations causing a defect in cytokinesis. In addition, certain phosphorylation site mutants induce a delay or block in cytokinesis when overexpressed. Together, these data identify novel interactions of a fission yeast GSK-3 homolog with elements of the cytokinesis machinery.     

Focal increases in [Ca]i may account for apparent low Ca sensitivity in swine carotid artery

Estimates of [Ca²⁺]_i sensitivity in intact smooth muscle are frequently obtained by measuring [Ca²⁺]_i with indicators such as aequorin or Fura-2. We investigated whether focal in increases in [Ca²⁺]_i could impair such measures of [Ca²⁺]_i sensitivity. Stimulation of swine carotid artery with 10 μM histamine increased aequorin estimated [Ca²⁺]_i, Fura-2 estimated [Ca²⁺]_i and Ca²⁺ sensitivity without significantly altering the aequorin/Fura-2 ratio (an estimate of [Ca²⁺]_i homogeneity). Subsequent inhibition of Na⁺/Ca²⁺ exchange by replacement of Na⁺ in the PSS with choline⁺ significantly increased aequorin-estimated [Ca²⁺]_i but only minimally increased Fura-2 estimated [Ca²⁺]_i, myosin light chain (MLC) phosphorylation and force. This resulted in a large increase in the aequorin/Fura-2 ratio, suggesting an increase in [Ca²⁺] inhomogeneity. Addition of 100 μM histamine to tissues in the choline⁺ buffer initially increased both aequorin and Fura-2 estimated [Ca²⁺]_i but after 10 min exposure both of the [Ca²⁺]_i estimates declined to pre-histamine levels. Histamine addition significantly increased MLC phosphorylation and force, indicating increased Ca²⁺ sensitivity, but the aequorin/Fura-2 ratio remained elevated and uncharged from pre-histamine values. These data show that under certain conditions, aequorin and Fura-2 can yield widely differing estimates of [Ca²⁺]_i, and thus can cause misleading assessments of Ca²⁺ sensitization mechanisms. These discrepancies may arise from inhomogeneous or focal increases in [Ca²⁺]_i which can be evaluated with the aequorin/Fura-2 ratio.