Autoradiograms: 35S and 32P

SDS-PAGE for 35S Immunoprecipitation and Immunoprecipitation Western Blotting

This report discusses recent methods of sample preparation and gel electrophoresis for ³⁵S immunoprecipitation (IP) and IP western blotting. In both methods, IP is used to obtain purified proteins, and sodium dodecyl sulfate polyacrylamide gel electrophoresis is used to separate the proteins on a gel. In ³⁵S IP, the proteins are radiolabeled and visualized on film by fluorography; in IP blotting, proteins are transferred onto nitrocellulose paper, and antibodies are used to detect specific proteins. A similar IP and SDS-PAGE method can be used for both procedures, but IP blotting has the potential advantages of improvement in sensitivity for low-abundance proteins and enhanced specificity for identification of proteins from a mixture. Some of the technical adaptations discussed here to facilitate IP blotting and avoid loss of beads or purified proteins may also be useful for ³⁵S IP.

SDS-PAGE for ³⁵S immunoprecipitation and immunoprecipitation western blotting

This report discusses recent methods of sample preparation and gel electrophoresis for (35)S immunoprecipitation (IP) and IP western blotting. In both methods, IP is used to obtain purified proteins, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) is used to separate the proteins on a gel. In (35)S IP, the proteins are radiolabeled and visualized on film by fluorography; in IP blotting, proteins are transferred onto nitrocellulose paper, and antibodies are used to detect specific proteins. A similar IP and SDS-PAGE method can be used for both procedures, but IP blotting has the potential advantages of improvement in sensitivity for low-abundance proteins and enhanced specificity for identification of proteins from a mixture. Some of the technical adaptations discussed here to facilitate IP blotting and avoid loss of beads or purified proteins may also be useful for (35)S IP.

Electrophoretic mobility shift assay (EMSA) for detecting protein-nucleic acid interactions

The gel electrophoresis mobility shift assay (EMSA) is used to detect protein complexes with nucleic acids. It is the core technology underlying a wide range of qualitative and quantitative analyses for the characterization of interacting systems. In the classical assay, solutions of protein and nucleic acid are combined and the resulting mixtures are subjected to electrophoresis under native conditions through polyacrylamide or agarose gel. After electrophoresis, the distribution of species containing nucleic acid is determined, usually by autoradiography of 32P-labeled nucleic acid. In general, protein-nucleic acid complexes migrate more slowly than the corresponding free nucleic acid. In this protocol, we identify the most important factors that determine the stabilities and electrophoretic mobilities of complexes under assay conditions. A representative protocol is provided and commonly used variants are discussed. Expected outcomes are briefly described. References to extensions of the method and a troubleshooting guide are provided.

Identification of genes preferentially induced by nerve growth factor versus epidermal growth factor in PC12 pheochromocytoma cells by means of representational difference analysis

Neurotrophins induce neuronal differentiation by binding to a subclass of ligand-stimulated protein tyrosine kinase receptors and activating signal transduction pathways that mediate altered patterns of gene expression. Nerve growth factor (NGF) induced differentiation of PC12 pheochromocytoma cells is one of the major model systems used to study neuronal differentiation in response to neurotrophins. Although epidermal growth factor (EGF) does not induce PC12 cell differentiation, NGF and EGF activate many of the same signal transduction pathways and induce transcription of many of the same genes in PC12 cells. We have now employed cDNA representational difference analysis to identify four genes (activity-regulated cytoskeletal protein, collagenase 1, plasminogen activator inhibitor-1, and VH6/MKP-3) as genes preferentially induced withing 4 hours by NGF in PC12 cells.

Methoxatin (PQQ) in guinea-pig neutrophils

PQQ, also called methoxatin, has been isolated from guinea-pig neutrophils. The organic cations diphenyleneiodonium (DPI) and diphenyliodonium (BPI) and the aromatic o-diamine 4,5-dimethylphenylenediamine (DIMPDA) sequester synthetic PQQ and inhibit its redox-cycling activity in a model system. Standards were made of adducts of tritiated PQQ with unlabeled DIMPDA and of unlabeled PQQ with tritiated DPI or DIMPDA. PQQ adducts were isolated from guinea-pig neutrophils with each of the tritiated inhibitors. They were separated and defined by high-performance liquid chromatography (HPLC). Tiron, a disodium benzene disulphonic acid, broke the DPI-PQQ adduct isolated from neutrophils and released free PQQ. Both DPI and DIMPDA, as well as BPI, blocked O2.- release by stimulated neutrophils. The blockade exerted by these inhibitors was released by the addition of PQQ to the cell suspensions. The data demonstrate the presence of PQQ in guinea-pig neutrophils and suggest that it has a possible role, direct or indirect, in the O2.(-)-producing respiratory burst.

Biosynthesis and secretion of rat salivary proteins by Xenopus laevis oocytes

Xenopus laevis oocytes injected with poly (A+) RNA isolated from rat parotid and submandibular glands synthesize and secrete salivary proteins. Amylase was identified in the media of cultured oocytes injected with rat parotid mRNA by size and immunoprecipitation with anti-human amylase serum. Secretion of the salivary proteins was detectable in the medium eight h following the parotid mRNA injection and continued in a time-dependent fashion for up to 96 h. In contrast to rat parotid slices in culture, which demonstrate a regulated pathway of secretion highly responsive to the secretagogue isoproterenol, secretion of salivary proteins by oocytes did not respond to the stimulation by isoproterenol. Though parotid mRNA is presumed to contain the templates encoding the regulated pathway of secretion, reconstitution of this pathway of secretion in oocytes was not observed in our experiments. Since Xenopus laevis oocytes secrete constitutively significant amounts of proteins when injected with salivary gland mRNA, they are a useful biological system for the analysis of secretion, processing, and function of salivary proteins.

Expression of extracellular matrix components is regulated by substratum

Reconstituted basement membranes and extracellular matrices have been demonstrated to affect, positively and dramatically, the production of milk proteins in cultured mammary epithelial cells. Here we show that both the expression and the deposition of extracellular matrix components themselves are regulated by substratum. The steady-state levels of the laminin, type IV collagen, and fibronectin mRNAs in mammary epithelial cells cultured on plastic dishes and on type I collagen gels have been examined, as has the ability of these cells to synthesize, secrete, and deposit laminin and other, extracellular matrix proteins. We demonstrate de novo synthesis of a basement membrane by cells cultured on type I collagen gels which have been floated into the medium. Expression of the mRNA and proteins of basement membranes, however, are quite low in these cultures. In contrast, the levels of laminin, type IV collagen, and fibronectin mRNAs are highest in cells cultured on plastic surfaces, where no basement membrane is deposited. It is suggested that the interaction between epithelial cells and both basement membrane and stromally derived matrices exerts a negative influence on the expression of mRNA for extracellular matrix components. In addition, we show that the capacity for lactational differentiation correlates with conditions that favor the deposition of a continuous basement membrane, and argue that the interaction between specialized epithelial cells and stroma enables them to create their own microenvironment for accurate signal transduction and phenotypic function.