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D M Engelman

Publications and source records attributed to D M Engelman.

At least 109 records · Page 6Linked to original sources
PubMed1981

The spontaneous insertion of proteins into and across membranes: the helical hairpin hypothesis.

We propose that the initial event in the secretion of proteins across membranes and their insertion into membranes is the spontaneous penetration of the hydrophobic portion of the bilayer by a helical hairpin. Energetic considerations of polypeptide structures in a nonpolar, lipid environment compared with an aqueous environment suggest that only alpha and 3(10) helices will be observed in the hydrophobic interior of membranes. Insertion of a polypeptide is accomplished by a hairpin structure composed of two helices, which will partition into membranes if the free energy arising from burying hydrophobic helical surfaces exceeds the free energy "cost" of burying potentially charged and hydrogen-bonding groups. We suggest, for example, that the hydrophobic leader peptide found in secreted proteins and in many membrane proteins forms one of these helices and is oriented in the membrane with its N terminus inside. In secreted proteins, the leader functions by pulling polar portions of a protein into the membrane as the second helix of the hairpin. The occurrence of all categories of membrane proteins can be rationalized by the hydrophobic or hydrophilic character of the two helices of the inserted hairpin and, for some integral membrane proteins, by events in which a single terminal helix is inserted. We propose that, because of the distribution of polar and nonpolar sequences in the polypeptide sequence, secretion and the insertion of membrane proteins are spontaneous processes that do not require the participation of additional specific membrane receptors or transport proteins.

Biological Transport↗
PubMed1981

Conformation of liquid N-alkanes.

The conformations of liquid n-alkanes have been studied using neutron scattering techniques to better understand the conformational forces present in membrane lipid interiors. We have studied hydrocarbon chains having lengths comparable to those found for esterified membrane lipid fatty acids, and find that the steric constraints of packing in the liquid state do not change the conformational distributions of hydrocarbon chains from those imposed by the intrachain forces present in the gas phase. It follows that the central region of membranes containing lipids in the disordered state should contain hydrocarbon chain conformations determined primarily by intrachain forces.

Alkanes↗
PubMed1981

Cholesteryl myristate conformation in liquid crystalline mesophases determined by neutron scattering.

The possible involvement of cholesteryl ester states in the development and persistence of atherosclerosis and the transport and storage of cholesteryl esters has led to questions concerning the organization and conformation of cholesteryl ester molecules in both pure phases and membranes. The experiments we report here were designed to measure the distance between the center of mass of the fatty acyl terminal methyl group and the center of mass of the three-carbon branched terminus of the cholesterol moiety at the opposite end of the molecule. The distance obtained is thus a gauge of cholesteryl ester conformation through the conformational range from a completely extended conformation to a U-shaped conformation. Neutron scattering experiments on partially deuterated samples of pure cholesteryl myristate in the crystalline, smectic, cholesteric, and isotropic phases indicate that the molecule is extended in each of these states. A discussion of specific molecular models consistent with these results and extension of these conclusions to other cholesteryl esters is included.

Cholesterol Esters↗
PubMed1980

Bacteriorhodopsin is an inside-out protein.

Neutron scattering is particulary useful when parts of a structure can be deuterated. From Halobacterium halobium we have obtained, by biosynthetic incorporation, purple membranes in which all of the valines or all of the phenylalanines are present in deuterated form. Difference Fourier techniques permit a general assessment of the distribution of valine and phenylalanine in projections of the purple membrane structure. These show that valine is distributed toward the periphery of a single bacteriorhodopsin molecule, whereas phenylalanine is distributed toward its center. We use the facts that the amino acid sequence is known and that much of it can be assigned to the alpha helices of the bacteriorhodopsin structure to interpret our results. Comparison of our maps with the distribution of valine and phenylalanine around alpha-helical perimetrs establishes the distribution of other amino acids and leads to the conclusion that the charged and polar groups of the bacteriorhodopsin molecule tend to lie at the molecular interior, away from contact with lipid, while the nonpolar surfaces are directed outward, making contact with the lipid regions. Thus, the protein is "inside-out" compared with the organization of soluble proteins.

Bacteriorhodopsins↗
PubMed1980

Path of the polypeptide in bacteriorhodopsin.

An attempt has been made to fit the amino acid sequence of bacteriorhodopsin to the three-dimensional density map of the molecule. First, seven segments of the sequence were selected as being probable transmembrane alpha helices. Then each of the 5040 possible ways of fitting these seven segments into the seven regions of helical density in the map were evaluated based on the criteria of connectivity of the nonhelical link regions, charge neutralization, and total scattering density per helix. A single model that may be experimentally tested emerged as the most probable.

Bacteriorhodopsins↗
PubMed1979

Substrate binding closes the cleft between the domains of yeast phosphoglycerate kinase.

Using small angle x-ray scattering from solutions of yeast phosphoglycerate kinase, we have measured the radius of gyration of the enzyme both in the presence and in the abscence of ligands. We find that the radius of gyration decreases by 1.09 +/- 0.34 A upon binding both substrates MgATP and 3-phosphoglycerate to form the ternary complex. Smaller decreases, at the limit of the precision of the measurement, were found for the separate binding of MgATP (0.30 +/- 0.50 A). Using computer modeling, it has been estimated that a substrate-induced cleft closure in phosphoglycerate kinase resulting from one lobe rotating 8-14 degrees relative to the other lobe lobe is consistent with this observed change in radius of gyration. We suggest, therefore, that the conformational change that results in the smaller radius of gyration for the ternary complex is a hinge motion of the two lobes which produces a closing of the cleft between the two lobes. The apparent similarity of the ligand-induced change in phosphoglycerate kinase to the cleft closure in hexokinase suggests that this kind of conformational change may prove to be a rather general kinase phenomenon (Bennett, W.S., and Steitz T.A. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4848-4852; Anderson, C.M., Zucker, F.H., and Steitz, T.A. (1979) Science 204, 375-380).

Adenosine Triphosphate↗
PubMed1979

Small angle X-ray scattering of dimeric yeast hexokinase in solution.

Small angle x-ray scattering measurements on dimeric yeast hexokinase B at pH 5.5 in acetate buffer yield a radius of gyration of 31.28 +/- 0.23 angstrom. This measured value is comparable to the radius of gyration of 31.5 angstrom calculated from the refined coordinates of the dimer in the BII crystal form. The hexokinase dimer found in the BI crystal form has a radius of gyration of 42 angstrom calculated from the atomic coordinates. Thus, the measured radius of gyration is consistent with the BII dimer being the predominant species in solution and rules out the existence of the BI dimer as a major species under these conditions.

Hexokinase↗
PubMed1979

Yeast hexokinase in solution exhibits a large conformational change upon binding glucose or glucose 6-phosphate.

Using small-angle X-ray scattering from solutions of yeast hexokinase, we have measured the radii of gyration of the monomeric B isozyme and its complexes with sugar substrates. We find that the radius of gyration decreases by 0.95 +/- 0.24 A upon binding glucose and 1.25 +/- 0.28 A upon binding glucose 6-phosphate. This observed reduction in radius of gyration in the presence of glucose is the same as that calculated from the coordinates of the high-resolution crystal structures of native hexokinase B and a glucose complex with hexokinase A. Thus, these measurements suggest that the dramatic closing of the slit between the two lobes of hexokinase observed in the crystal structures (Bennett, W.S., & Steitz, T.A. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4848--4852) occurs in solution when either glucose or glucose 6-phosphate is bound.

Glucose↗
PubMed1978

The planar distributions of surface proteins and intramembrane particles in Acholeplasma laidlawii are differentially affected by the physical state of membrane lipids.

We have studied the influence of changes in lipid organization on the planar distribution of two classes of membrane proteins: integral proteins which have amino groups exposed to labelling at the membrane surface by the biotinavidin-ferritin procedure, and those proteins which penetrate the lipid bilayer sufficiently to be seen as intramembranous particles by freeze-fracture electron-microscopy. When the membranes are examined at temperatures below the lipid phase transition, the first class is dispersed and the second patched. At temperatures in the middle of the transition range, both classes are patched. At temperatures just above the phase transition the first class is dispersed and the second patched, and at temperatures well above the transition both classes are dispersed. Freeze-etch studies of avidin-ferritin-labeled membranes confirmed that the distribution seen by the labeling and the freeze-fracture techniques coexist in single membranes. Thus, there exist two distinct classes of membrane proteins with differential organizational responses to the lipid state.

Acholeplasma laidlawii↗
PubMed1978

X-ray and neutron small-angle scattering studies of the complex between protein S1 and the 30-S ribosomal subunit.

X-ray neutron solution scattering experiments have been done to investigate the influence of the binding of ribosomal protein S1 on the conformation of the 30-S ribosomal subunit of Escherichia coli. The following conclusions were made. 1. The alterations (if any) in conformation of the non-S1 parts of the 30-S subunit induced by S1 binding are too small to be detected (less than 0.1 nm change in radius of gyration). 2. The center of gravity of protein S1 bound to the 30-S subunit is quite far from the center of gravity of the particle (approximately 7.5 nm).

Escherichia coli↗