Mutants helix

LD spectra Rb. capsulatus (mutant, helix exchange), 289, 295f ... 

Gerber, D. and Y. Shai. 2000. Insertion and organization within membranes of the 8-endotoxin pore-forming domain, helix 4-loop-helix 5, and inhibition of its activity by a mutant helix 4 peptide. J. Biol. Chem. 275 23602-23607. 

The elegant genetic studies by the group of Charles Yanofsky at Stanford University, conducted before the crystal structure was known, confirm this mechanism. The side chain of Ala 77, which is in the loop region of the helix-turn-helix motif, faces the cavity where tryptophan binds. When this side chain is replaced by the bulkier side chain of Val, the mutant repressor does not require tryptophan to be able to bind specifically to the operator DNA. The presence of a bulkier valine side chain at position 77 maintains the heads in an active conformation even in the absence of bound tryptophan. The crystal structure of this mutant repressor, in the absence of tryptophan, is basically the same as that of the wild-type repressor with tryptophan. This is an excellent example of how ligand-induced conformational changes can be mimicked by amino acid substitutions in the protein. 

One long a helix connects the two domains (purple). Thermostable mutants of this protein were constructed by introducing disulfide bridges at three different places (yellow). 

Mutant YE-2 of Rhizobium meliloti excretes a mixture of soluble polysaccharides that include a complex succinoglycan having a branched octasaccharide repeat as well as a simple galactoglucan (22) having a linear disaccharide repeat.102 In contrast to the case of the succinoglycan, oriented fibers of the potassium salt of 22 have yielded good X-ray data and its three-dimensional structure has been established.39 The polymer forms a two-fold helix of pitch... 

The E. coli M41 mutant CPS (46) has a complex chemical sequence. Its repeating unit is an anionic hexamer a tetrasaccharide -A-B-C-D- in the main chain and a disaccharide -F-E- side chain, E attached to C (Table II). Polycrystalline and oriented fibers of the sodium salt of 46 have produced good diffraction data, with reflections up to 3 A resolution. Careful X-ray analysis60 has shown that the polymer forms a left-handed, smooth and sinuous, 2-fold helix of pitch 30.4 A. As shown in Fig. 39a, the main chain is fairly close to the helix axis. A notable observation is that side chain E-F, turned up toward the non-re-... 

Fig. 13 DNA-protein CT reactions. The DNA-bound protein, methyltransferase Hhal (mutant Q237W), flips a base out of the DNA double helix and inserts a trytophan side chain leaving the /r-stack largely unperturbed. This intercalated trytophan moiety transfers an electron to [Ru(bpy )(dppz)(phen)]3+, generated by flash quench, over 50 A away. Adapted from [164]... 

Hunt I think that E—Q mutant you discovered inadvertently could have differential effects on the binding of cyclin, because that helix is a major point of cyclin/Cdc2 contact. We certainly have mutants which map in different places that affect the relative strength of binding of cyclins A and B. 

A160G mutant (open circles) against temperature. (A) Alal03 (C-D loop), (B) Ala39 (helix B), 168 (helix F), (C) Ala228, 233 (C-terminal a-helix), D-F (transmembrane a-helices). From Ref. 192 with permission. 

In addition, Zn2+ was shown to inhibit dopamine uptake in a mutant containing an engineered tridentate zinc site, in which the i-4 site from His3757.60, Met3717.56, was replaced with histidine, whereas the introduction of histidines at the i-2, i-3, and i-5 position did not increase Zn2+ affinity (29). In contrast, histidines at positions i+2, i+3, and i+4 all resulted in potent inhibition of dopamine uptake by Zn2+. The incorporation of these data in a model of secondary structure provides evidence for an a-helical configuration of the extracellular portion of TM7, as well as the absence of well-defined secondary structure between positions 3757.6o and 37 97.64 (Fig. IB), thereby suggesting an approximate boundary between the C-terminal end of the helix and the beginning of EL4 (29). 

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