Intracellular localization of the mRNAs of argininosuccinate synthetase and argininosuccinate lyase around liver mitochondria, visualized by high-resolution in situ reverse transcription-polymerase chain reaction

Argininosuccinate Synthetase is Reversibly Inactivated by S-Nitrosylation in Vitro and in Vivo

Prior studies have demonstrated that the substrate for NO synthesis, l-arginine, can be regenerated from the NOS co-product l-citrulline. This requires the sequential action of two enzymes, argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). AS activity has been shown to be rate-limiting for high output NO synthesis by immunostimulant-activated cells and represents a potential site for metabolic control of NO synthesis. We now demonstrate that NO mediates reversible S-nitrosylation and inactivation of AS in vitro and in lipopolysaccharide-treated cells and mice. Using a novel mass spectrometry-based method, we show that Cys-132 in human AS is the sole target for S-nitrosylation among five Cys residues. Mutagenesis studies confirm that S-nitrosylation of Cys-132 is both necessary and sufficient for the inhibition of AS by NO donors. S-nitroso-AS content is regulated by cellular glutathione levels and selectively influences NO production when citrulline is provided to cells as a protosubstrate of NOS but not when l-arginine is provided. A phylogenetic comparison of AS sequences suggests that Cys-132 evolved as a site for post-translational regulation of activity in the AS in NOS-expressing species, endowing NO with the capacity to limit its own synthesis by restricting arginine availability.

Localization of transcripts corresponding to the major allergen from olive pollen (Ole e I) by electron microscopic non-radioactive in situ RT-PCR

In situ reverse transcription-PCR of mRNAs corresponding to the olive major allergen (Ole e I) has been tested at the ultrastructural level in mature olive pollen. The transcripts were present in the cytoplasm of both the vegetative and the generative cells, frequently associated to ribosomes in the endoplasmic reticulum. No labeling was detected in the pollen wall, nor in vacuoles, lipid bodies, plastids or mitochondria. Localization of the major olive allergen at ultrastructural level showed the protein present mainly in the lumen of the endoplasmic reticulum vesicles or pockets scattered in the cytoplasm, and in the outer region of the pollen exine. The results confirm the rough endoplasmic reticulum as the cell system involved in both the synthesis and storage of this protein. This is the first report of in situ RT-PCR on plant material at the ultrastructural level. The method described for mRNA amplification and detection is confirmed as a valuable tool for studying gene expression in plant material.

Cytoarchitecture and physical properties of cytoplasm: volume, viscosity, diffusion, intracellular surface area

Classical biochemistry is founded on several assumptions valid in dilute aqueous solutions that are often extended without question to the interior milieu of intact cells. In the first section of this chapter, we present these assumptions and briefly examine the ways in which the cell interior may depart from the conditions of an ideal solution. In the second section, we summarize experimental evidence regarding the physical properties of the cell cytoplasm and their effect on the diffusion and binding of macromolecules and vesicles. While many details remain to be worked out, it is clear that the aqueous phase of the cytoplasm is crowded rather than dilute, and that the diffusion and partitioning of macromolecules and vesicles in cytoplasm is highly restricted by steric hindrance as well as by unexpected binding interactions. Furthermore, the enzymes of several metabolic pathways are now known to be organized into structural and functional units with specific localizations in the solid phase, and as much as half the cellular protein content may also be in the solid phase.

Substrate channeling

Substrate channeling is the process in which the intermediate produced by one enzyme is transferred to the next enzyme without complete mixing with the bulk phase. This process is equivalent to a microcompartmentation of the intermediate, although classic diffusion occurs simultaneously to varying extents in many of these cases. This microcompartmentation and other factors of channeling provide many potential biological advantages. Extensive examples of channeling can be found in the cited reviews. The choice of methods to detect and characterize substrate channeling depends extensively on the type of enzyme associations involved, the constants of the system, and, to some extent, the mechanism of channeling. Thus it is important to distinguish stable, dynamic, and catalytically induced enzyme associations as well as recognize different mechanisms of substrate channeling. We discuss the principles, experimental details, and limitations and precautions of five rather general methods. These use measurements of transient times, isotope dilution or enhancement, competing reaction effects, enzyme buffering kinetics, and transient-state kinetics. These encompass methods applicable to studies in vitro, in situ, and in vivo. None of these methods is applicable to all systems. They are also susceptible to artifacts without proper attention to precautions. Transient-state kinetic methods clearly excel in elucidating molecular mechanisms of channeling. However, they are often not the best method for initial detection and characterization of the process and they are not applicable to many complex systems. Several other methods that have been successful in indicating substrate channeling are briefly described.

Regulatory signals in messenger RNA: determinants of nutrient–gene interaction and metabolic compartmentation

Nutrition has marked influences on gene expression and an understanding of the interaction between nutrients and gene expression is important in order to provide a basis for determining the nutritional requirements on an individual basis. The effects of nutrition can be exerted at many stages between transcription of the genetic sequence and production of a functional protein. This review focuses on the role of post-transcriptional control, particularly mRNA stability, translation and localization, in the interactions of nutrients with gene expression. The effects of both macronutrients and micronutrients on regulation of gene expression by post-transcriptional mechanisms are presented and the post-transcriptional regulation of specific genes of nutritional relevance (glucose transporters, transferrin, selenoenzymes, metallothionein, lipoproteins) is described in detail. The function of the regulatory signals in the untranslated regions of the mRNA is highlighted in relation to control of mRNA stability, translation and localization and the importance of these mRNA regions to regulation by nutrients is illustrated by reference to specific examples. The localization of mRNA by signals in the untranslated regions and its function in the spatial organization of protein synthesis is described; the potential of such mechanisms to play a key part in nutrient channelling and metabolic compartmentation is discussed. It is concluded that nutrients can influence gene expression through control of the regulatory signals in these untranslated regions and that the post-transcriptional regulation of gene expression by these mechanisms may influence nutritional requirements. It is emphasized that in studies of nutritional control of gene expression it is important not to focus only on regulation through gene promoters but also to consider the possibility of post-transcriptional control.