Precursors folding

The reactive domains are cleaved from the precursor at a highly conserved repeat domain with the sequence EEKKN. Identification of small quantities of peptides with extended or shortened termini has been taken as an indicator for the involvement of unspecific proteases in the cleavage of the precursor. Structural studies of a Cl-Tl construct derived from the precursor protein using NMR indicate that the domains within the precursor fold independently from each other and that no interdomain interactions are detectable on long-term scale. ... 

B. Correlation between Precursor Folding and Loss of Translocation Competence... 

The overwhelming body of evidence demonstrates that SecB binds within the mature moiety of MBP, not the signal peptide. However, two predictions of this axiom have not been met (1) synthesis of mMBP should cause interference (it does not) and (2) mMBP synthesized in vitro should form detectable complexes with SecB (it does not). A resolution of this apparent paradox lies in the ability of the signal peptide to influence the rate of precursor folding and therefore indirectly affect SecB binding. 

In the production of cytoplasmic ribosomes in human cells, one portion of the 45S rRNA precursor becomes the 18S rRNA that, complexed with proteins, forms the small 40S ribosomal subunit (Fig. 14.15, circle 4). Another segment of the precursor folds back on itself and is cleaved, forming 28S rRNA, hydrogen-bonded to the 5.8S rRNA. The 5S rRNA, transcribed from nonnucleolar genes, and a number of proteins complex with the 28S and 5.8S rRNAs to form the 60S ribosomal subunit (Fig. 14.15, circle 5). The ribosomal subunits migrate through the nuclear pores. In the cytoplasm, the 40S and 60S ribosomal subunits interact with mRNA, forming the 80S ribosomes on which protein synthesis occurs. 

Anilines react with ct-haloacetophenones to give 2-arylindoles. In a typical procedure an W-phenacylaniline is heated with a tw o-fold excess of the aniline hydrobromide to 200-250°C[1]. The mechanism of the reaction was the subject of considerable investigation in the 1940s[2]. A crucial aspect of the reaction seems to be the formation of an imine of the acetophenone which can isomerize to an aldimine intermediate. This intermediate apparently undergoes cyclization more rapidly (path bl -> b2) than its precursor (Scheme 7.3). Only with very reactive rings, e.g, 3,5-dimethoxyaniline, has the alternative cydiz-ation (path al a2) to a 3-arylindole been observed and then only under modified reaction conditions[3],... 

Figure 11.7 Schematic diagram of the structure of chymotrypsin, which is folded into two antiparallel p domains. The six p strands of each domain are red, the side chains of the catalytic triad are dark blue, and the disulfide bridges that join the three polypeptide chains are marked in violet. Chain A (green, residues 1-13) is linked to chain B (blue, residues 16-146) by a disulfide bridge between Cys 1 and Cys 122. Chain B is in turn linked to chain C (yellow, residues 149-245) by a disulfide bridge between Cys 136 and Cys 201. Dotted lines indicate residues 14-15 and 147-148 in the inactive precursor, chmotrypsinogen. These residues are excised during the conversion of chymotrypsinogen to the active enzyme chymotrypsin.

The efficient uptake of precursor proteins depends on their presentation in a translocation competent state. This is maintained in vivo by the specific interaction with a highly conserved group of proteins, the heat-shock or stress related proteins (hps70s). These act as molecular chaperones and interact with the proteins to maintain them in a correctly folded state, a process which is ATP dependent. 

Matthews DA, Smith WW, Ferre RA, Condon B, Budahazi G, Sisson W, Villafranca JE, Janson CA, McElroy HE, Gribskov CL et al (1994) Structure of human rhinovirus 3C protease reveals a trypsin-like polypeptide fold, RNA-binding site, and means for cleaving precursor polyprotein. CeU 77 761-771... 

Particle Generation Rate. The particle generation rate was calculated from the concentration of k-fold precursor particles assuming Muller coagulation kinetics 2.) as well as including propagation terms. 

Number of k-fold Precursor Particles. Dynamic differential equations were written for the concentration of the k-fold precursors to account for birth and death by coagulation, growth by propagation, and the formation of primary precursors by homogeneous nucleation. There... 

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