Conformational energy analysis of the substitution of Val for Gly 233 in a functional region of platelet GPIbα in platelet-type von Willebrand disease

Platelet-type von Willebrand disease (PT-vWD) is an autosomal dominant bleeding disorder in which patient platelets exhibit an abnormally increased binding of circulating von Willebrand factor (vWF). We have recently shown that this abnormality is associated with a point mutation resulting in substitution of Val for Gly 233 in platelet membrane glycoprotein Ibα (GPIbga), a major component of the platelet (GPIb/IX receptor for vWF. To investigate the effect of this substitution on the three-dimensional structure of this region of the protein, we have generated the allowed (low energy) conformations of the region of the GPIα protein containing residues 228–238 (with 5 residues on either side of the critical residue 233) with Gly 233 (wild type) and Val 233 (PT-vWD) using the computer program ECEPP (Empirical Conformational Energies of Peptides Program). The wild-type sequence is Tyr-Val-Trp-Lys-Gln-Gly-Val-Asp-Val-Lys-Ala. We find that the Gly 233-containing peptide can exist in two low energy conformers. The lowest energy conformer is a structure containing a β-turn at Gln 232-Gly 233 while the alternative conformation is an amphipathic helical structure. Only the amphipathic helical structure is allowed for the Val 233-containing peptide which contains a hydrophobic ‘face’ consisting of Val 229, Val 233 and Val 236 and another hydrophilic surface composed of such residues as Lys 231 and Asp 235. No such surfaces exist for the lowest energy bend conformer for the Gly 233-containing peptide, but do exist in the higher energy helical structure. The amphiphatic surfaces in the 228–238 region of the Val 233-containing GPIbα protein may associate strongly with complementary surfaces during vWF binding to the GPIb/IX receptor complex and may help explain heightened association of vWF with this receptor in PT-vWD.     

Slipped-strand mispairing: a major mechanism for DNA sequence evolution

Simple repetitive DNA sequences are a widespread and abundant feature of genomic DNA. The following several features characterize such sequences: (1) they typically consist of a variety of repeated motifs of 1-10 bases--but may include much larger repeats as well; (2) larger repeat units often include shorter ones within them; (3) long polypyrimidine and poly-CA tracts are often found; and (4) tandem arrangements of closely related motifs are often found. We propose that slipped-strand mispairing events, in concert with unequal crossing- over, can readily account for all of these features. The frequent occurrence of long tandem repeats of particular motifs (polypyrimidine and poly-CA tracts) appears to result from nonrandom patterns of nucleotide substitution. We argue that the intrahelical process of slipped-strand mispairing is much more likely to be the major factor in the initial expansion of short repeated motifs and that, after initial expansion, simple tandem repeats may be predisposed to further expansion by unequal crossing-over or other interhelical events because of their propensity to mispair. Evidence is presented that single-base repeats (the shortest possible motifs) are represented by longer runs in mammalian introns than would be expected on a random basis, supporting the idea that SSM may be a ubiquitous force in the evolution of the eukaryotic genome. Simple repetitive sequences may therefore represent a natural ground state of DNA unselected for coding functions.      

Complete sequences of the rRNA genes of Drosophila melanogaster

In this, the first of three papers, we present the sequence of the ribosomal RNA (rRNA) genes of Drosophila melanogaster. The gene regions of D. melanogaster rDNA encode four individual rRNAs: 18S (1,995 nt), 5.8S (123 nt), 2S (30 nt), and 28S (3,945 nt). The ribosomal DNA (rDNA) repeat of D. melanogaster is AT rich (65.9% overall), with the spacers being particularly AT rich. Analysis of DNA simplicity reveals that, in contrast to the intergenic spacer (IGS) and the external transcribed spacer (ETS), most of the rRNA gene regions have been refractory to the action of slippage-like events, with the exception of the 28S rRNA gene expansion segments. It would seem that the 28S rRNA can accommodate the products of slippage-like events without loss of activity. In the following two papers we analyze the effects of sequence divergence on the evolution of (1) the 28S gene "expansion segments" and (2) the 28S and 18S rRNA secondary structures among eukaryotic species, respectively. Our detailed analyses reveal, in addition to unequal crossing-over, (1) the involvement of slippage and biased mutation in the evolution of the rDNA multigene family and (2) the molecular coevolution of both expansion segments and the nucleotides involved with compensatory changes required to maintain secondary structures of RNA.      

Comparison of acid phosphatase ACP1 variants by isoelectric focusing and conventional electrophoresis: identification of three new alleles, ACP1*N, ACP1*P and ACP1*S

Three new alleles of human red cell acid phosphatase (ACP1) have been identified by comparison with previously reported variants using three different electrophoretic techniques. Family data are available on all the variants and show genetic transmission of the rare alleles ACP1*N, ACP1*P and ACP1*S. Further evidence of a rare allele demonstrating reversed 'A' activity is also described. The report documents the need to use several electrophoretic techniques to characterize new or rare variants.     

Population structure of Alaskan and Siberian indigenous communities

Genetic data consisting of 14 red cell antigen groups and 11 serum and erythrocytic protein marker systems from four Eskimo populations of the Norton Sound area of Alaska are reported. The population structure of these four groups is analyzed both separately and within the larger context by comparison to 15 other circumpolar groups. These analyses reveal a good fit between genetic structure, geographic distribution, linguistic affiliation, and the ethnohistory of the region.     

Advantages of sucrose-dependent extracellular polysaccharide production to lactic acid bacteria in sucrose-rich environments

G.A. DYKES, I. GEORNARAS AND A. VON HOLY. 1995. Some possible advantages of sucrosedependent extracellular polysaccharide production to Lactobacillus L191 from baker's yeast were investigated. A ca log 1 plaque-forming units ml^-1decrease in attachment of bacteriophage AB1 to cells of Lactobacillus L191 grown in the presence of sucrose as compared to cells grown in the absence of sucrose was noted. On the other hand, a ca log 1 colony-forming units cm^-2increase in attachment of Lactobacillus L191 to stainless steel surfaces when grown in the presence of sucrose compared to the absence of sucrose was observed. It was concluded that sucrose-dependent extracellular polysaccharide may provide a series of individually small but jointly synergistic selective advantages to strains producing it.     

Structural studies on porcine brain tubulin in extended sheets

Structural studies have been conducted on porcine brain tubulin, assembled into microtubule-related structures, by electron microscopy in conjunction with optical diffraction and image reconstruction techniques. By minimizing background noise and sample damage, we have improved the resolution on negatively stained samples, extending the data from the previous limit of a 42 Å layer line to an additional layer line at 21 Å. The new reflections confirm the basic surface lattice proposed from the earlier studies and extend the structural features that can be assigned to individual tubulin molecules. Data are obtained for microtubules in standard buffers for both the helical form and flat sheets of up to 13 protofilaments. When zinc is added to the preparations, sheets with more than 13 protofilaments are formed and the extended lattices provide more reflections on both the 42 Å and 21 Å layer lines, as well as the equator. The lattice in the presence of zinc differs considerably from the normal lattice, with adjacent protofilaments staggered by 21 Å, compared to the staggering of adjacent filaments of about 10 Å in the absence of zinc. There is also a distinct pairing of adjacent protofilaments in the zinc-induced sheets. Initial studies with the Unwin-Henderson method on unstained zinc-tubulin sheets suggest that the adjacent protofilaments may be related by a dyad axis, either perpendicular or parallel to the protofilament axes.