N terminus, in proteins

RCSB Protein Data Bank 1EX7, enzyme with a non-acetylated N terminus in a complex with GMP [22]) [7-10, 13, 19, 18, 22]... 

Single transmembrane helix Ca2+ receptor N terminus in vesicle SNARE proteins, C termini in vesicle Integral membrane protein Integral membrane protein, Cl- transporter Integral membrane protein Integral membrane protein 13 subunits H+ generator... 

The protein-chemical characterisation revealed a homotetrameric enzyme with a molecular mass of 362 kDa, an isoelectric point of pi 6.16 and a blocked N-terminus in Edman degradation studies [353], In biochemical studies we investigated the effect of free and chelated first-row transition metal ions (Cu2+, Ni2+, Zn2+ and Co2+) on SuSy activity [355]. Further experiments on the binding behaviour of SuSy in immobilized metal ion affinity chromatography (I MAC) gave an insight into the topography of sucrose synthase from rice... 

Fig. 6 21 Schematic views of G-protein coupled receptors, (a) Cross-sectional view of a G-protein coupled receptor demonstrating the N terminus in the extracellular space, seven transmembrane domains and the C terminus in the cytoplasm. (b) A view of the transmembrane domains of a G-protein coupled receptor from above demonstrating how amino acid side chains can form a precise ligand-binding site. In this example the ligand-binding site is shown to be formed by a side chain carboxylate group, two side chain amino groups and three side chain hydroxyls.

The problem of N-terminal variants in recombinant proteins is not uncommon. E. coli synthesizes proteins with a formylated methionine at the N terminus. In vivo, E. coli often removes N-formyl methionine with the action of a deformylase followed by methionine amino peptidase. This removal is not always exact and neighboring amino acids in the peptide chain influence the removal.130 This can yield recombinant products lacking a number of encoded amino acids at the N terminus. For smaller proteins, these product-related impurities generally are detected and quantitated by RP-HPLC. However, large proteins differing by only one or two N-terminal amino acids may be difficult to resolve by RP-HPLC. In these instances, peptide mapping by RP-HPLC is a valuable tool. 

The unfolding behaviors of the authentic and recombinant forms of goat a-lactalbumin are remarkably different, although both forms have an identical three-dimensional structure. The recombinant form was found to be l.lkcalmol-1 less stable than the authentic form, and the recombinant form unfolded at a ninefold faster rate than the authentic form. The destabilization and unfolding-rate acceleration were due to the presence of an extra methionine residue at the N-terminus in the recombinant protein. 

A separate study investigated the function of the N-terminal region of the EHV coat protein in determining the geometry of the particle. It was postulated, on the basis of the position of the N terminus in the high-resolution structure of mature virions, that deletion of the first 31 residues would result in assembly of a particle with T = 1 symmetry containing 60... 

The plasma membrane of the cell is a lipid bilayer sheet in which membrane-bound proteins are embedded. Steps 4B-6B of Figure 1.21 illustrate some events in the production of a membrane-bound protein. After synthesis of the protein, the ribosome on which it was formed dissociates from the membrane but the protein remains bound to the membrane (Step 4B). This binding is mediated by a short stretch of lipophilic amino acids that may occur near the C terminus, as shown in Figure 1.21, or near the N terminus in the case of other proteins. Subsequently, part of the ER membrane forms a bud that breaks off (Step 5B) to form a secretory vesicle (Step 6B). The continued association of the entire membrane-bound protein during the budding process and during subsequent events is maintained by the special lipophilic sequence. Eventually, the secretory vesicle fuses with the plasma membrane in a process that resembles a reversal of Steps 4B-6B. After completion of the insertion of the membrane-bound protein into the plasma membrane, its N terminus is in contact with the extracellular fluid and its C terminus is in contact with the cytoplasm, at least for the protein depicted in Figure 1.21. 

Proteins with N-Terminus in the Exoplasmic Space (Type IV-B) The large family of G protein-coupled receptors, all of which contain seven transmembrane a helices, constitute the most numerous type IV-B proteins, whose N-termlnus extends into the exoplasmic space. In these proteins, the hydrophobic a helix closest to the N-termlnus often is followed by a cluster of positively charged amino acids, similar to a type III signal-anchor sequence. As a result, the first a helix Inserts the nascent chain into the translocon with the N-terminus extending into the lumen (see Figure 16-13e). As the chain is elongated, it is Inserted into the ER membrane... 

The amino acid sequence of a fragment containing a reactive cysteine residue was reported by Harris (86) and, later, that of another peptide containing a second cysteine residue by Twu et al. (406,407). The absence of a free N-terminus in the protein was demonstrated in other studies (378,404) and subsequently found to be resulting from blockage by an acyl group (1-2). Tryptic peptides of the enzyme were partly characterized by Butler et al. (378,379). 

Leuconostoc mesenteroides (4 3), Pseudomonas aeruginosa (4 4), Streptococcus mutans (435), and Bacillus stearothermophilus (486,437). The protein from the latter species was purified by Kolb and Harris (436) and found to have a free N-terminus in contrast to the liver and yeast alcohol dehydrogenases. The bacterial protein was, therefore, submitted to sequence analysis in an automatic sequenator which revealed the 45 first residues (IS). These are listed in Table XIV. It is clear that the structure of the bacterial enzyme is distantly related to those of the yeast and mammalian enzymes, but few residues are identical in all proteins at equivalent positions (Table XIV). Further aspects of this relationship are discussed in Section II,D. 

Figure 1.—Continued. Computer-generated molecular model of synthetic pore proteins. (C) Energy-optimized parallel tetramers of T4CaIVS3 (upper) and T4M28 (lower). C is the end view with the N terminus in front (89). Residues are colored according to hydrophobicity see previous page.

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