Cytosolic protein

Squalene monooxygenase, an enzyme bound to the endoplasmic reticulum, converts squalene to squalene-2,3-epoxide (Figure 25.35). This reaction employs FAD and NADPH as coenzymes and requires Og as well as a cytosolic protein called soluble protein activator. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. 

Parvalbumin (Fig 1) is a cytosolic protein expressed in fast-twitch skeletal muscles and in the nervous system. In muscles, parvalbumin controls the relaxation process. In the CNS, parvalbumin, expressed in a subpopulation of GABAergic neurons, is correlated with their firing rates, protecting the cells from Ca2+ overload. 

These cytosolic proteins contain five EF-hand domains and are able to translocate to the plasma membrane upon calcium binding [5]. In addition to the EF-hand domains, these proteins also have a hydrophobic glycine/proline-rich domain, important for their translocation to the membrane. To date five members of this... 

Parvalbumin is a cytosolic protein expressed mainly in skeletal muscles and brain. 

The general transport routes of proteins are well established (Fig. la) [1]. Initially, every protein is synthesized at cytoplasmic ribosomes. Its destination within the cell is then determined by transport signals. If the nascent chain does not contain transport signals, it becomes a cytosolic protein. 

Fujiki, M. Vemer, K. (1993). Coupling of cytosolic protein synthesis and mitochondrial protein import in yeast. J. Biol. Chem. 268, 1914-1920. 

In vivo studies in animals suggest that endosulfan may disrupt normal reproductive hormone levels in male animals, but that it is not an endocrine disrupter in females. Persistent depressed testicular testosterone was seen in male rats after intermediate duration oral exposures to endosulfan. In ovariectomized female rats, orally administered endosulfan did not induce normal development of female reproductive tissues, and in female mice and immature female rats, acute parenteral exposure to endosulfan did not affect several endocrine-related end points. In vitro studies have evaluated endosulfan for estrogen receptor (ER) and cytosolic protein binding affinity, ER-mediated reporter gene expression, estrogenic induction of cell proliferation, and alteration of relative abundance of active estradiol metabolites. Overall, in vitro evidence in favor of endosulfan estrogenicity indicates relatively weak potency compared to 17[3-estradiol. Apparently contradictory results were reported in different... 

Figure26-2. Biosynthesis of squalene, ubiquinone, dolichol, and other polyisoprene derivatives. (HMG, 3-hydroxy-3-methylglutaryl x, cytokinin.) A farnesyl residue is present in heme a of cytochrome oxidase. The carbon marked with asterisk becomes C or C,2 in squalene. Squalene synthetase is a microsomal enzyme all other enzymes indicated are soluble cytosolic proteins, and some are found in peroxisomes.

Once bilirubin enters the hepatocytes, it can bind to certain cytosolic proteins, which help to keep it solubilized prior to conjugation. Ligandin (a family of glutathione S-transferases) and protein Y are the involved proteins. They may also help to prevent efflux of bilirubin back into the blood stream. 

Translocation is believed to occur posttranslation-ally, after the matrix proteins are released from the cytosolic polyribosomes. Interactions with a number of cytosolic proteins that act as chaperones (see below) and as targeting factors occur prior to translocation. 

Synthesis of the transferrin receptor (TfR) and that of ferritin are reciprocally linked to cellular iron content. Specific untranslated sequences of the mRNAs for both proteins (named iron response elements) interact with a cytosolic protein sensitive to variations in levels of cellular iron (iron-responsive element-binding protein). When iron levels are high, cells use stored ferritin mRNA to synthesize ferritin, and the TfR mRNA is degraded. In contrast, when iron levels are low, the TfR mRNA is stabilized and increased synthesis of receptors occurs, while ferritin mRNA is apparently stored in an inactive form. This is an important example of control of expression of proteins at the translational level. 

Gugger, E.T. and Erdman, J.W., Intracellular (3-carotene transport in bovine liver and intestine is not mediated by cytosolic proteins, J. Nutr, 126, 1470, 1996. 

FIGURE 10.3 Structure of the BHTOH-KLH conjugate utilized for raising polyclonal antibodies. Western blots from 2D gels of cytosolic proteins isolated from mouse lung epithelial cells. The blot on the right is from cells incubated with BHT-QM and the blot on the left is from untreated cells. Immunoreactive proteins were identified by electrospray LC-MS analysis of the tryptic digests. Source From Ref. 39, with permission from the American Chemical Society. 

Figure 3 A hydrophobic permeant must negotiate through a complex series of diffu-sional and thermodynamic barriers as it penetrates into a cell. The lipid and protein compositions and charge distribution of the inner and outer leaflets of the membrane lipid bilayer can play limiting roles, particularly at the tight junction. Depending upon the permeant s characteristics, it may remain within the plasma membrane or enter the cytoplasm, possibly in association with cytosolic proteins, and partition into cytoplasmic membranes.

Ayers, N. A., Kapas, L. Krueger, J. M. (1996). Circadian variation of nitric oxide synthase activity and cytosolic protein levels in rat brain. Brain Res. 707, 127-30. 

Hoffmann, R, Ji, H., Moritz, R. L., Connolly, L. M., Frecklington, D. F., Layton, M. J., Eddes, J. S., Simpson, R. J. (2001). Continuous free-flow electrophoresis separation of cytosolic proteins from the human colon carcinoma cell line LIM 1215 a non two-dimensional gel electrophoresis-based proteome analysis strategy. Proteomics 1(7), 807. 

Witzmann FA et al. Differential expression of cytosolic proteins in the rat kidney cortex and medulla preliminary proteomics. Electrophoresis 1998 19 2491-2497. 

Yoshikawa T, Okada N, Oda A et al (2008) Development of amphiphilic y-PGA-nanoparticle based tumor vaccine potential of the nanoparticulate cytosolic protein delivery carrier. Biochem Biophys Res Commun 366 408 -13... 

Martin, T. F. J. and Walent, J. H. A new method for cell per-meabilization reveals a cytosolic protein requirement for Ca2+-activated secretion in GH3 pituitary cells. /. Biol. Chem. 264 10299-10308,1989. 

---