The central problem in muscle research today is to understand the molecular mechanism of the force-generating event. There is a large body of well-established evidence that the HMM regions of myosin are the elements responsible for the generation of contractile force. The current models on the muscle contraction, Huxley (1969, 1984), Huxley-Simmons (1971) and helix-coil transition (1971, 1979), are discussed in this review. Within muscle, the chemical event (hydrolysis of ATP) is coupled with a mechanical event (force generation due to structural changes). Two discrete models are equally possible, either the mechanochemical events are directly coupled with S-1 of myosin or indirectly coupled (S-1 hydrolyzes ATP but force is generated with the S-2 region of the myosin rod). Recent biochemical evidence strongly suggests that a substantial amount of melting within the S-2 hinge domain (over 200A in length) occurs upon activation of muscle. These findings favor the "helix-coil" transition model as the most likely mechanism.
Evolutionary features of the immunoglobulin variable region heavy chain (VH) multigene family were discussed with special reference to the concerted evolution. In particular, a phylogenetic tree for the DNA sequences of 16 mouse and five human germline genes was constructed. This tree indicates that all genes in this family have undergone substantial evolutionary divergence. The most closely related genes so far identified in the mouse genome seem to have diverged about 6 million years (MY) ago, whereas the most distantly related genes diverged about 300 MY ago. This suggests that gene duplication caused by unequal crossing-over or gene conversion occurs very slowly in this gene family. The rate of occurrence of gene duplication in the VH gene family has been estimated to be 5×10-7 per gene per year, which seems to be at least about 100 times lower than that for the rRNA gene family. This low rate of concerted evolution in the VH gene family helps retain intergenic genetic variability that in turn contributes to antibody diversity.
Vacuoles are generally believed to present storage compartments for inorganic ions and a variety of organic compounds. However, studies on the mechanism of vacuolar compartmentation were indirect. and rather confusing. In yeast Saccharomyces cerevisiae, vacuoles are the largest organelles, occupying about 25% of the total cell volume, and they are postulated to function as lysosomes and as a stomge compartment of basic amino acids, S-adenosylmethionine and polyphophates. There must exist some specific transport mechanisms in the vacuolar membranes. Recently, we established a procedure for preparing right-side-out vacuolar membrane vesicles of high purity from cells of the yeast, Saccharomyces cerevisiae, and showed that the vesicles catalyze active transport of ten amino acids and Ca2+ which are driven by an electrochemical potential difference of protons formed by ATP hydrolysis. Studies on ATPase activity on vacuolar membrane vesicles indicated the presence of a new type of H+-ATPase, which is different from mitochodrial and plasma membrane H+-ATPases. This vacuolar membrane ATPase was partially purified and showed entirely different subunit structure from other two ATPases. These results suggest that vacuoles have their own energy tranducing mechanism and active transport systems, and play active roles on metabolic regulation of the cells.