Biomedical subjects

James L Baber

Publications and source records attributed to James L Baber.

4 recordsLinked to original sources

PubMed2006

Measurement of 15N relaxation in the detergent-solubilized tetrameric KcsA potassium channel.

A set of TROSY-HNCO (tHNCO)-based 3D experiments is presented for measuring (15)N relaxation parameters in large, membrane-associated proteins, characterized by slow tumbling times and significant spectral overlap. Measurement of backbone (15)N R (1), R (1rho), (15)N-{(1)H} NOE, and (15)N CSA/dipolar cross correlation is demonstrated and applied to study the dynamic behavior of the homotetrameric KcsA potassium channel in SDS micelles under conditions where this channel is in the closed state. The micelle-encapsulated transmembrane domain, KcsA(TM), exhibits a high degree of order, tumbling as an oblate ellipsoid with a global rotational correlation time, tau(c) = 38 +/- 2.5 ns, at 50 degrees C and a diffusion anisotropy, Dparallel/Dperpendicular = 0.79+/-0.05, corresponding to an aspect ratio a/b >/= 1.4. The N- and C-terminal intracellular segments of KcsA exhibit considerable internal dynamics (S (2) values in the 0.2-0.45 range), but are distinctly more ordered than what has been observed for unstructured random coils. Relaxation behavior in these domains confirms the position of the C-terminal helix, and indicates that in SDS micelles, this amphiphilic helix does not associate into a stable homotetrameric helical bundle. The relaxation data indicate the absence of elevated backbone dynamics on the ps-ns time scale for the 5-residue selectivity filter, which selects K(+) ions to enter the channel.

Anisotropy↗

PubMed2004

Characterization of phospholipid mixed micelles by translational diffusion.

The concentration dependence of the translational self diffusion rate, D (s), has been measured for a range of micelle and mixed micelle systems. Use of bipolar gradient pulse pairs in the longitudinal eddy current delay experiment minimizes NOE attenuation and is found critical for optimizing sensitivity of the translational diffusion measurement of macromolecules and aggregates. For low volume fractions Phi (Phi\\ le 15% v/v) of the micelles, experimental measurement of the concentration dependence, combined with use of the D (s)= D (o)(1-3.2lambdaPhi) relationship, yields the hydrodynamic volume. For proteins, the hydrodynamic volume, derived from D (s) at infinitely dilute concentration, is found to be about 2.6 times the unhydrated molecular volume. Using the data collected for hen egg white lysozyme as a reference, diffusion data for dihexanoyl phosphatidylcholine (DHPC) micelles indicate approximately 27 molecules per micelle, and a critical micelle concentration of 14 mM. Differences in translational diffusion rates for detergent and long chain phospholipids in mixed micelles are attributed to rapid exchange between free and micelle-bound detergent. This difference permits determination of the free detergent concentration, which, for a high detergent to long chain phospholipid molar ratio, is found to depend strongly on this ratio. The hydrodynamic volume of DHPC/POPC bicelles, loaded with an M2 channel peptide homolog, derived from translational diffusion, predicts a rotational correlation time that slightly exceeds the value obtained from peptide (15)N relaxation data.

Biophysics↗

PubMed2002

Molecular basis of sequence-specific single-stranded DNA recognition by KH domains: solution structure of a complex between hnRNP K KH3 and single-stranded DNA.

To elucidate the basis of sequence-specific single-stranded (ss) DNA recognition by K homology (KH) domains, we have solved the solution structure of a complex between the KH3 domain of the transcriptional regulator heterogeneous nuclear ribonucleoprotein K (hnRNP K) and a 10mer ssDNA. We show that hnRNP K KH3 specifically recognizes a tetrad of sequence 5'd-TCCC. The complex is stabilized by a dense network of methyl-oxygen hydrogen bonds involving the methyl groups of three isoleucine residues and the O2 and N3 atoms of the two central cytosine bases. Comparison with the recently solved structure of a specific protein-ssDNA complex involving the KH3 and KH4 domains of the far upstream element (FUSE) binding protein FBP suggests that the amino acid located five residues N-terminal of the invariant GXXG motif, which is characteristic of all KH domains, plays a crucial role in discrimination of the first two bases of the tetrad.

Amino Acid Motifs↗

PubMed2002

Structure and dynamics of KH domains from FBP bound to single-stranded DNA.

Gene regulation can be tightly controlled by recognition of DNA deformations that are induced by stress generated during transcription. The KH domains of the FUSE-binding protein (FBP), a regulator of c-myc expression, bind in vivo and in vitro to the single-stranded far-upstream element (FUSE), 1,500 base pairs upstream from the c-myc promoter. FBP bound to FUSE acts through TFIIH at the promoter. Here we report the solution structure of a complex between the KH3 and KH4 domains of FBP and a 29-base single-stranded DNA from FUSE. The KH domains recognize two sites, 9-10 bases in length, separated by 5 bases, with KH4 bound to the 5' site and KH3 to the 3' site. The central portion of each site comprises a tetrad of sequence 5'd-ATTC for KH4 and 5'd-TTTT for KH3. Dynamics measurements show that the two KH domains bind as articulated modules to single-stranded DNA, providing a flexible framework with which to recognize transient, moving targets.

Amino Acid Sequence↗