Ribonuclease extracellular

Leucine residues 2, 5, 7, 12, 20, and 24 of the motif are invariant in both type A and type B repeats of the ribonuclease inhibitor. An examination of more than 500 tandem repeats from 68 different proteins has shown that residues 20 and 24 can be other hydrophobic residues, whereas the remaining four leucine residues are present in all repeats. On the basis of the crystal structure of the ribonuclease inhibitor and the important structural role of these leucine residues, it has been possible to construct plausible structural models of several other proteins with leucine-rich motifs, such as the extracellular domains of the thyrotropin and gonadotropin receptors. 

Many extracellular proteins like immunoglobulins, protein hormones, serum albumin, pepsin, trypsin, ribonuclease, and others contain one or more indigenous disulfide bonds. For functional and structural studies of proteins, it is often necessary to cleave these disulfide bridges. Disulfide bonds in proteins are commonly reduced with small, soluble mercaptans, such as DTT, TCEP, 2-mercaptoethanol, thioglycolic acid, cysteine, etc. High concentrations of mercaptans (molar excess of 20- to 1,000-fold) are usually required to drive the reduction to completion. 

Nurnberger, T., Abel, S., Jost, W. Glund, K. (1990). Induction of an extracellular ribonuclease in cultured tomato cells upon phosphate starvation. Plant Physiology 92, 970-6. 

Support for this picture can be found in a study of the two closely related mesophilic proteins barnase and binase. These two extracellular ribonucleases are highly similar in structure, stability, and sequence, differing by only seventeen out of 110 amino acids. Serrano et al. (1993) introduced each mutation present in binase into barnase and measured... 

Bamase is an extracellular ribonuclease wUch is often used as a model ft>r protein folding. It is a idative y small protein (110 amino acids, 12.4 kDa). Bamase contans three tryptophan residues (Figure 16.44), one of which is located close to histidine-18. Examination of the three single-tryptophan mutants of bamase provided an interesting example of energy transfer between tryptophan residues and the quenching effects of the nearby histidine. "... 

The association of bamase, an extracellular ribonuclease, with its intracellular inhibitor, barstar, provides a particularly well-characterized example of electrostatically steered protein-protein encounter. The association rate is very fast (about 10 -10 at 50 mM ionic strength), and mutation and ionic-strength-dependence studies clearly show the influence of electrostatic interactions. Brownian dynamics simulations are able to reproduce the ionic-strength dependence of the rate for the wild-type proteins and the rates for wild-type and 11 mutants at 50 mM ionic strength to within a factor of 2. These simulations provide insight into the structure of the encounter (transition state) complex in which barstar tends to be shifted from its position in the bound complex towards the guanine binding loop on bamase. 

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