Mutation addition

The evolution of antiviral resistance for viruses discussed in this chapter (HIV, HBV, HCV, and Influenza virus) shares some common features. Replication in vivo results in the generation of viral variation and selection of preexisting viruses from the population occurs under particular conditions. This will only happen when the escaping viruses have a sufficient level of both resistance and RC. In most cases, the resistance level subsequently increases further by the gradual acquisition of further mutations. Additional compensatory mutations then accumulate that help to restore full RC in the third stage. 

Fig. 4.5.4 Identification of mutations in the transferrin protein by neuraminidase treatment. Unusual patterns in the IEF of serum transferrin might lead to pitfalls in CDG diagnostics. These varying patterns are often due to mutations of charged amino acids in the protein backbone of the transferrin molecule, which might lead, for example, to an accumulation of trisialo transferrin bands (lane 3, indicated by a question mark). Further investigations are carried out by cleaving off charged sialic acid monosaccharide moieties from transferrin-linked oligosaccharides by neuraminidase treatment, followed by IEF and transferrin antibody staining. In the case of protein mutations, additional bands below (lane 4) or above (not shown) the desialylated transferrin form appear...

During experiments, coupling is observed through non-additive mutational effects. Two mutations are considered additive if the combined change in fitness is equal to the sum of the individual contributions of each mutation. Additivity implies a smooth landscape that can be easily... 

The 12 possible types of nucleotide substitution can be treated differently (assuming nonsymmetry of change, e.g., the frequency of A to C does not equal that for C to A) or treated equally, or any combination of these substitutions can be grouped. One obvious division of base substitutions is to treat transitions (changes of purine to purine or pyrimidine to pyrimidine) separately from transversions (change of purine to pyrimidine or vice versa). Insertion/deletion events can also be treated as a separate type of mutation. Additionally, nucleotide substitutions can be preferentially treated by a combination of position and mutation (e.g., transversions occurring in the first and second codon positions). 

To this author s knowledge, mutations responsible for human galactosi-alidosis have been described only in Japanese patients. One mutation was reported for the late-onset form of the disease prevalent in Japan (Shimmoto et al., 1990 Suzuki et al., 1991). A substitution from G (the third base from the 5 donor site of intron 7) to A appears to cause skipping of exon 7 during processing of the transcript. All nine Japanese patients with the late-onset form had this mutation. Additional mutations have since been reported in the literature (Takano et al., 1991 Shimmoto et al, 1993). 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Ethylene oxide has been shown to produce mutagenic and cytogenic effects in a variety of test systems (226). An increased frequency of chromosomal aberrations in peripheral lymphocytes of monkey exposed to ethylene oxide for 104 weeks has been reported (240). In mice, it is an effective inducer of chromosome breaks leading to dominant-lethal mutations. In addition, ethylene oxide has been shown to induce heritable effects in the heritable translocation test conducted in mice exposed to ethylene oxide by inhalation (241,242). In this study, male mice were exposed to ethylene oxide ranging from 165 to 300 ppm for 6 h per day 5 or 7 days/week for 8.5 weeks. Ethylene oxide has also been shown to bind to proteins (243) as well as to DNA (244). Several studies on ethylene oxide-exposed workers have demonstrated an increased incidence of chromosomal aberrations and sister chromatid exchanges the relevance of such effects to human health evaluation is currendy uncertain. 

In summary, structural studies of Ras and Gq with GTP-yS and a transition state analog have illuminated the catalytic mechanism of their GTPase activity, as well as the mechanism by which GTP hydrolysis is stimulated by GAP and RGS. In addition, these structural studies have shown how tumor-causing mutations affect the function of Ras and Gq. 

Src tyrosine kinase contains both an SH2 and an SH3 domain linked to a tyrosine kinase unit with a structure similar to other protein kinases. The phosphorylated form of the kinase is inactivated by binding of a phosphoty-rosine in the C-terminal tail to its own SH2 domain. In addition the linker region between the SH2 domain and the kinase is bound in a polyproline II conformation to the SH3 domain. These interactions lock regions of the active site into a nonproductive conformation. Dephosphorylation or mutation of the C-terminal tyrosine abolishes this autoinactivation. 

Both types of mutations have been made in T4 lysozyme. The chosen mutations were Gly 77-Ala, which caused an increase in Tm of 1 °C, and Ala 82-Pro, which increased Tm by 2 °C. The three-dimensional structures of these mutant enzymes were also determined the Ala 82-Pro mutant had a structure essentially identical to the wild type except for the side chain of residue 82 this strongly indicates that the effect on Tm of Ala 82-Pro is indeed due to entropy changes. Such effects are expected to be additive, so even though each mutation makes only a small contribution to increased stability, the combined effect of a number of such mutations should significantly increase a protein s stability. 

---