Genetic insertion of nuclear spin labels in the lac repressor

Two-dimensional 1H, 15N NMR investigation of uniformly 15N-labeled lac repressor headpiece

15N uniformly labeled lac repressor and lac repressor headpiece were prepared. 15N NMR spectra of lac repressor were shown resolution inadequate for detailed study while the data showed that the 15N labeled N-terminal part of the protein is quite suitable for this type of study allowing future investigation of the specific interaction of the lac repressor headpiece with the lac operator. We report here the total assignment of proton 1H and nitrogen 15NH backbone resonances of this headpiece in the free state. Assignments of the 15N resonances of the protein were obtained in a sequential manner using heteronuclear multiple quantum coherence (HMQC), relayed HMQC nuclear Overhauser and relayed HMQC-HOHAHA spectroscopy. More than 80 per cent of residues were assigned by their 15NH(i)-N1H(i + 1) and 15NH(i)-N1H(i - 1) connectivities. Values of the 3JNH alpha splitting for 39 of the 51 residues of the headpiece were extracted from HMQC and HMQC-J. The observed 15NH(i)-C beta H cross peaks and the 3JNH alpha coupling constants values are in agreement with the three alpha-helices previously described [Zuiderweg, E.R.P., Scheek, R.M., Boelens, R., van Gunsteren, W.F. and Kaptein, R., Biochimie 67, 707 (1985)]. The 3JNH alpha coupling constants can be now used for a more confident determination of the lac repressor headpiece. From these values it is shown that the geometry of the ends of the second and third alpha-helices exhibit deviation from the canonical alpha-helix structure. On the basis of NOEs and 3JNH alpha values, the geometry of the turn of the helix-turn-helix motif is discussed.

Direct electrophilic fluorination of tyrosine in dermorphin analogues and its effect on biological activity, receptor affinity and selectivity

In a preliminary communication we reported [(Tetrahedron Lett. 31, 619 (1990)] that acetyl hypofluorite can be used efficiently to introduce fluorine regiospecifically (ortho to OH) into the phenolic ring of tyrosine-containing peptides. This procedure has been applied to the fluorination of a number of mu-selective opioid peptides derived from dermorphin. While the procedure can be used even when the side chains of Arg, Lys, and Tyr are left unprotected, the sulfoxide of a Met(O)-containing analogue was oxidized to sulfone faster than fluorination of the phenolic ring. This method can also be used when the peptide is attached to Merrifield resin. Thus, Tyr(3-F)-D-Ala-Phe-Gly-NH2 and Tyr(3-F)-D-Arg-Phe-Lys-NH2 (F-DALDA) have been prepared, purified, and characterized. Affinities of these fluorinated peptides for both mu- and delta- opioid receptors are reduced (two- to nine-fold) relative to their nonfluorinated analogues, but their selectivity for mu-opioid receptors is not significantly altered. Similarly, the in vitro biological potencies (GPI and MVD assays) of the fluorinated analogues are reduced (two- to seven-fold) relative to their nonfluorinated parent peptides. Thus, F-DALDA, which has high affinity (Ki mu = 15.2 nM) and selectivity (Ki delta/Ki mu = 5390) for mu-opioid receptors, has potential use in biochemical studies which utilize 19F or 18F- labeled compounds.

Use of nonsense suppression to generate altered proteins

The use of suppressed nonsense mutations to generate altered proteins can greatly simplify studies in which a large number of defined mutant proteins are sought. If site-directed mutagenesis is used to generate specific mutations, than for every amber (UAG) mutation constructed, as many as 13 different amino acids can be inserted at the corresponding site in the protein. This allows a rapid screening of many altered proteins for those with interesting properties. Once identified, the interesting substitutions can be regenerated by missense changes, to avoid some of the potential problems of the method. Nonsense suppression has been used to generate more than 3300 amino acid replacements in the E. coli lac repressor, and close to 250 amino acid substitutions in E. coli thymidylate synthase.

Solution NMR studies of intact lambda repressor

Using a combination of two and one-dimensional NMR spectroscopy, it is shown that in the intact bacteriophage lambda repressor, the N-terminal domain assumes the same global structure as when it remains isolated. It is further shown that the N-terminal domain is only loosely attached to the C-terminal domain in the intact repressor.