Lipochondria and the light response of Aplysia giant neurons

The ultrastructure, absorbance, and elemental content of lipochondria present in the cytoplasm of Aplysia giant neurons have been investigated before and after 30–1,200 sec doses of white light at intensities which produce saturated light responses. The effects of exposure to the calcium ionophore A-23187 and to EGTA were also examined. The lipochondria of nonilluminated neurons are membrane-bound, and contain lipids, protein, Na, K, Mg, Ca, Si, Cl, Br, P, and a pigment which is probably β-carotene. The cytoplasm appeared to have little pigment. When neurons were illuminated for 20 min, 60–70% of the lipochondria showed marked ultrastructural alterations, the most notable being the appearance of membranous material. Earlier changes which occur after 30 sec of illumination include the appearance of paracrystalline arrays and mottling. Less than 10% of lipochondria in nonilluminated neurons have a similar appearance. These effects were greatly enhanced in illuminated neurons exposed to the calcium ionophore or EGTA. In nonilluminated neurons, the ionophore also produced ultrastructural changes. In frozen specimens, the calcium content of the most electron dense lipochondria of illuminated neurons was reduced. Other elements which were counted were also reduced. The lipochondria are the main intracellular site of photopigment. They may also act as an intracellular source for calcium which, as the accompanying paper indicated, may mediate phototransduction in Aplysia neurons.     

Re-appraisal of the nature of the pigment produced by Aeromonas salmonicida

Abstract Aeromonas salmonicida grown aerobically in the presence of tyrosine produces a dark-brown pigment, previously considered to be of a melanin type. However, this pigment is very water-soluble in contrast to melanin's solubility. In addition, no evidence for the presence of key early reactions and intermediates of the melanogensis pathway has been obtained. Radioisotopic labelling experiments, spectral data and chromatographic analyses support the conclusion that pigment biosynthesis, from tyrosine by Aeromonas salmonicida differs substantially from that of melanogenesis, and strongly suggest that the first intermediate is 2,4-dihydroxyphenylalanine and not 3,4-dihydroxyphenylalanine.     

Novel shuttle plasmid vehicles for Escherichia-Streptococcus transgeneric cloning

A novel plasmid vector that is able to replicate both in Escherichia coli and in Streptococcus sanguis is described. This 9.2-kb plasmid, designated pVA856, carries Cm^r, Tc^r and Em^r determinants that are expressed in E. coli. Only the Em^r determinant is expressed in S. sanguis. Both the Cm^r and the Tc^r of pVA856 may be insertionally inactivated. This plasmid affords several different cleavage-ligation strategies for cloning in E. coli followed by subsequent introduction of chimeras in to S. sanguis. In addition, we have modified a previously described E. coli-S. sanguis shuttle plasmid [pVA838; Macrina et al., Gene 19 (1982) 345–353], so that it is unable to replicate in S. sanguis. The utility of such a plasmid for cloning and selecting sequences enabling autonomous replication in S. sanguis is demonstrated.     

Light-dependent chemical modification of thylakoid membrane proteins with carboxyl-directed reagents

Spinach chloroplast thylakoid membranes were chemically modified with membrane penetrating reagents reactive toward protein carboxyl groups, a carbodiimide and the nucleophiles [¹⁴C]glycine ethyl ester or [³H]serotonin. The reagents, being weak bases, were accumulated within the inner aqueous space in the light, due to the low pH inside. Both the accumulation and the low pH stimulating effect on the carbodiimide activation step contributed to a greater labeling in the light compared to dark, and uncouplers inhibited most of the light-dependent increase. Hence, it is likely that the proteins showing the light-dependent, uncoupler-sensitive labeling have those parts located within the inner aqueous space or within the membrane itself. While many membrane proteins which separated on sodium dodecyl sulfate-polyacrylamide gels (12.5–25% gradient) showed some increased labeling in the light, the most conspicuous were the four polypeptides of the chlorophyll $\text{a}\text{b}$ light-harvesting complex. The light-harvesting complex was purified from dark- and light-treated, labeled membranes. The resultant preparation showed about a sixfold, light-dependent, uncoupler-sensitive labeling increase compared to dark conditions. Polypeptides near 6 and 8 kdalton showed light-dependent, uncoupler-resistent increases in carboxyl group modification, which could be due to localized acidic conditions near sites of proton release.     

Adrenocortical cyclic GMP-dependent protein kinase: purification, characterization, and modification of its activity by calmodulin, and its relationship with steroidogenesis

Cyclic GMP-dependent protein kinase has been purified to apparent homogeneity from bovine adrenal cortex and its presence in the rat adrenal cortex has been demonstrated. Sucrose density sedimentation studies indicated that the M_r of the enzyme was 145,000. This protein was composed to two identical subunits each with M_r of 75,000. The enzyme molecule was asymmetric with a frictional coefficient of 1.54, Stokes radius of 53.5 Å and a sedimentation coefficient of 6.5. The enzyme self-phosphorylated and the stoichiometry of cyclic GMP binding was two molecules per holoenzyme. Calmodulin or troponin C markedly stimulated the apparent maximal velocity of cyclic GMP-dependent protein kinase without affecting its basal activity. This effect of protein modulators was independent of calcium. Sucrose density gradient studies indicated that the stimulatory effect of calmodulin was due to its interaction with histones. An interaction of calmodulin with the enzyme was not observed. The steroidogenic potential of cyclic GMP and its analogs correlated closely with their ability to stimulate cyclic GMP-dependent protein kinase; the order of potency for both activities was 8-bromocylic GMP > cyclic GMP > N²-monobutyryl cyclic GMP > N², O²-dibutyryl cyclic GMP. In each case, calmodulin enhanced the cyclic GMP-dependent protein kinase activity for histone phosphorylation. These results indicate that although cyclic GMP is the primary regulator of cyclic GMP-dependent protein kinase, other modulator proteins such as calmodulin could act as additional regulators of the phosphorylation of substrate proteins. In addition, the demonstration of cyclic GMP-dependent protein kinase in rat adrenal glands, and the results with cyclic GMP and its analogs relating to their activation of protein kinase and steroidogenesis are consistant with the concept that cyclic GMP is one of the mediators of adrenal steroidogenesis.     

Improved Method for Determination of Respiring Individual Microorganisms in Natural Waters

A method is reported that combines the microscopic determinations of specific, individual, respiring microorganisms by the detection of electron transport system activity and the total number of organisms of an estuarine population by epifluorescence microscopy. An active cellular electron transport system specifically reduces 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT) to INT-formazan, which is recognized as opaque intracellular deposits in microorganisms stained with acridine orange. In a comparison of previously described sample preparation techniques, a loss of >70% of the counts of INT-reducing microorganisms was shown to be due to the dissolution of INT-formazan deposits by immersion oil (used in microscopy). In addition, significantly fewer fluorescing microorganisms and INT-formazan deposits, both ≤0.2 μm in size, were found for sample preparations that included a Nuclepore filter. Visual clarity was enhanced, and significantly greater direct counts and counts of INT-reducing microorganisms were recognized by transferring microorganisms from a filter to a gelatin film on a cover glass, followed by coating the sample with additional gelatin to produce a transparent matrix. With this method, the number of INT-reducing microorganisms determined for a Chesapeake Bay water sample was 2-to 10-fold greater than the number of respiring organisms reported previously for marine or freshwater samples. INT-reducing microorganisms constituted 61% of the total direct counts determined for a Chesapeake Bay water sample. This is the highest percentage of metabolically active microorganisms of any aquatic population reported using a method which determines both total counts and specific activity.     

Studies on the activity of brown adipose tissue in suckling,pre-obese, obob mice

The properties and activity of brown adipose tissue have been investigated in suckling, pre-obese, $\text{ob}\text{ob}$ mice in order to determine whether decreased thermogenesis in the tissue precedes the development of obesity in this mutant. At 14 days of age there was no difference between the $\text{ob}\text{ob}$ and normal animals in the total amount of interscapular brown adipose tissue, and the DNA content, protein content, and cytochrome oxidase activity of the tissue were similar in the two groups of mice. Respiration rates of brown adipose tissue mitochondria in the presence of albumin were, however, greater in the normal than the $\text{ob}\text{ob}$ animals, although after the addition of GDP to recouple the mitochondria there was no difference between the two groups. The mitochondrial membrane potential, measured with [³H]methyltriphenylphosphonium, was less affected by exogenous GDP in $\text{ob}\text{ob}$ mice than in normal animals. GDP binding to brown adipose tissue mitochondria, an index of the proton conductance pathway, was much greater in normal than in $\text{ob}\text{ob}$ mice at both 10 and 14 days of age; the decreased GDP binding in the mutant animals was found to result from a reduction in the number of binding sites. It is concluded that brown adipose tissue mitochondria of pre-obese $\text{ob}\text{ob}$ mice are more tightly coupled than those of normal siblings, and that the activity of the ‘thermogenic’ proton conductance pathway is lower in the mutant animals. A decrease in thermogenesis in brown adipose tissue is therefore an early event in the development of the $\text{ob}\text{ob}$ mouse and precedes the appearance of obesity.     

Light-dependent conformational change at rhodopsin's cytoplasmic surface detected by increased susceptibility to proteolysis

Rhodopsin in bovine photoreceptor disk membranes was subjected to limited proteolysis by thermolysin, removing twelve amino acids from rhodopsin's carboxyl terminus. (1) The rate of proteolysis is significantly faster with rhodopsin following exposure to light than with unbleached rhodopsin, provided that the incubation conditions (pH, temperature) favor the formation of metarhodopsin II. (2) If the disk membranes are illuminated under conditions in which metarhodopsin I is the predominant photoproduct (pH 8.5, 0°C), no increase in the rate of proteolysis is observed compared to unilluminated membranes. (3) The light-induced increase in the rate of proteolysis is transient: it slowly decays in the dark to the original rate found for unbleached rhodopsin. The enhanced susceptibility to proteolysis appears to measure a conformational change at rhodopsin's cytoplasmic surface which is first exhibited at the metarhodopsin II stage. This and possibly other light-dependent changes may allow rhodopsin to mediate its signal as a light-receptor protein by binding to and activating certain rod cell enzymes.