Stereochemical studies on cyclic peptides: Detailed energy minimization studies on hydrogen bonded all-trans cyclic pentapeptide backbones

Conformational studies have been carried out on hydrogenbonded all-trans cyclic pentapeptide backbone. Application of a combination of grid search and energy minimization on this system has resulted in obtaining 23 minimum energy conformations, which are characterized by unique patterns of hydrogen bonding comprising of β- and γ-turns. A study of the minimum energy conformationsvis-a-vis non-planar deviation of the peptide units reveals that non-planarity is an inherent feature in many cases. A study on conformational clustering of minimum energy conformations shows that the minimum energy conformations fall into 6 distinct conformational families. Preliminary comparison with available X-ray structures of cyclic pentapeptide indicates that only some of the minimum energy conformations have formed crystal structures. The set of minimum energy conformations worked out in the present study can form a consolidated database of prototypes for hydrogen bonded backbone and be useful for modelling cyclic pentapeptides both synthetic and bioactive in nature. This is part XV of the series. Part XIV in this series is Ramakrishnanet al 1987.     

Steroid hormone responsiveness of a family of closely related mouse proviral elements

Regulation of the mouse sex-limited protein (Slp) gene is unusual in that hormone response is conferred by the 5′ LTR of an upstream inserted provirus, dubbed the imposon (imp1). In a search for additional genes whose regulation has been affected by retrotransposition events, we isolated two partial proviral elements by stringent screening of a mouse genomic library. One clone (imp2) contained a portion of the envelope gene and a 3′ LTR that was nearly identical to the 3′ LTR of imp1; this similarity extended to insertion into a B1 repetitive element. The second proviral clone (imp3) contained a 5′ LTR and associated coding sequences, but lacked its 3′ LTR; the LTR of imp3 differed by 12% from the imp1 sequence. To assess potential hormone response, proviral enhancer regions cloned into reporter vectors were tested in transfection. The imp2 enhancer was similar in behavior to imp1, conferring both androgen and glucocorticoid induction in one fragment context and an androgen-specific response in another. In contrast, the imp3 enhancer allowed high expression in the absence of hormone and was less responsive to steroids in general and androgen in particular. These three proviral elements define a small family of steroid responsive proviruses in the mouse genome, and at least one member has had a lasting impact on an endogenous gene's regulation. Received: 29 April 1997 / Accepted: 14 July 1997     

Seven New Mutations in hMSH2, an HNPCC Gene, Identified by Denaturing Gradient-Gel Electrophoresis

Hereditary nonpolyposis colorectal cancer (HNPCC) is a relatively common autosomal dominant cancer-susceptibility condition. The recent isolation of the DNA mismatch repair genes (hMSH2, hMLH1, hPMS1, and hPMS2) responsible for HNPCC has allowed the search for germ-line mutations in affected individuals. In this study we used denaturing gradient-gel electrophoresis to screen for mutations in the hMSH2 gene. Analysis of all the 16 exons of hMSH2, in 34 unrelated HNPCC kindreds, has revealed seven novel pathogenic germ-line mutations resulting in stop codons either directly or through frameshifts. Additionally, nucleotide substitutions giving rise to one missense, two silent, and one useful polymorphism have been identified. The proportion of families in which hMSH2 mutations were found is 21%. Although the spectrum of mutations spread at the hMSH2 gene among HNPCC patients appears extremely heterogeneous, we were not able to establish any correlation between the site of the individual mutations and the corresponding tumor spectrum. Our results indicate that, given the genomic size and organization of the hMSH2 gene and the heterogeneity of its mutation spectrum, a rapid and efficient mutation detection procedure is necessary for routine molecular diagnosis and presymptomatic detection of the disease in a clinical setup.     

APC mutation in the alternatively spliced region of exon 9 associated with late onset familial adenomatous polyposis

Germ-line mutations in the adenomatous polyposis coli (APC) gene are responsible for familial adenomatous polyposis (FAP). Genotype-phenotype correlation studies in patients with FAP have demonstrated associations of certain variants of the disease with mutations at specific sites within the APC gene. In a large FAP family, we identified a frameshift mutation located in the alternatively spliced region of exon 9. Phenotypic studies of affected family members showed that the clinical course of FAP was delayed, with gastrointestinal symptoms and death from colorectal carcinoma occurring on average 25 and 20 years later than usual, respectively. The numbers of colorectal adenomas differed markedly among affected individuals and the location of colorectal cancer lay frequently in the proximal colon. Our findings suggest that the exon 9 mutation identified in the pedigree is associated with late onset of FAP. The atypical phenotype may be explained by the site of the mutation in the APC gene. Analysis of the APC protein product indicated that the exon 9 mutation did not result in a detectable truncated APC protein. Given the location of the mutation within an alternatively spliced exon of APC, it is conceivable that normal APC proteins are produced from the mutant allele by alternative splicing.     

Localization of a gene for human α-galactosidase B (=N-Acetyl-α-D-Galactosaminidase) on chromosome 22

Summary The localization of the structural gene for human -galactosidase B (=N-acetyl--galactosaminidase) was investigated by means of man-Chinese hamster and man-mouse somatic cell hybrids. The hybrid clones were analyzed for chromosomes and for a large number of known enzyme markers. The lysates of the hybrid cells were treated with Sepharose-coupled antihuman -galactosidase B and the activity of the adsorbed enzyme was measured on the Sepharose beads as N-acetyl--galactosominidase. The results show that the structural gene for human -galactosidase B is situated on chromosome 22, and that there is no structural relationship between human -galactosidase A and human -galactosidase B.     

Effects of dynamic impacts on human bones

Flat bones of human skeleton were subjected to dynamic indentation with ball indenters. The impacted surface was studied under high magnification and also by using the technique of multiple beam interferometry. The impulse caused the pile up of material at a little distance from the edge of the indent. The diameter of indent is found to increase as fourth root of the energy of impact. Bone structure also has the tendency to minimize the damage caused by external forces. There was about 90% recovery in deformation in the depth of indents due to internal stresses created inside the bone by the impact.     

Localization of HLA on the short arm of chromosome 6

Summary A detailed marker gene study in a large Dutch kindred segregating for a reciprocal translocation between the chromosomes 6 and 20, t(6;20) (p21;p13), revealed a close linkage between the HLA genes and the breakpoint on the short arm of 6. During this study an apparent peak lod score of 2.9 was obtained at a recombination value of 0.05 for a linkage between HLA and the breakpoint, indicating that the chromosomal region, carrying the HLA genes, is situated near the breakpoint in band 6p21 close to the transition to 6p22.     

Orthogonal beta beta motifs in proteins.

A super-secondary structural motif comprising two orthogonally oriented beta-strands connected by short linking segments of less than or equal to 5 residues has been identified from a data set of 65 independent protein crystal structures. Of the 42 examples from 14 proteins, a vast majority have only a single residue as the linking element. Analysis of the conformational angles at the junction reveals that the recently described type VIII beta-turn occurs frequently at the connecting hinge, while the type II beta-turn is also fairly common.