Biomembranes最新文献

The light-induced formation of the photosynthetic membranes (greening) in y-1 mutant of Chlamydomonas reinhardi requires synthesis of new proteins which become incorporated into the growing membranes. It has been shown previously (Eytan and Ohad, 1970) that proteins synthesized by both chloroplast and cytoplasmic ribosomes concur in the formation of functional photosynthetic membranes, indicating the presence of a mechanism permitting the specific transfer of membrane proteins synthesized in the cytoplasm into the chloroplast. Transfer of such proteins cannot yet be identified by the usual biochemical techniques unless they become part of the growing photosynthetic membranes. However, it is possible to follow their synthesis and translocation between the different cellular compartments by use of quantitative electron microscopic radioautography. In the present work, the radioautographic grain distribution among chloroplast, chloroplast membrane, nucleus, mitochondria, and the remainder of the cytoplasm (cytosol) was carried out following short radioactive pulse-labeling and chase during greening of dark-grown mutants in the presence or absence of protein synthesis inhibitors. The results indicate that transport of some of the proteins of cytoplasmic origin to their final location within the chloroplast is at least partially controlled by concomitant synthesis of proteins by the chloroplast ribosomes.

The role of reactive SH groups (presumably in proteins) of the apical plasma membrane in transepithelial Na+ transport was studied in the isolated urinary bladder of the toad. On the basis of assays for TCA-soluble SH compounds (e.g., glutathione, methionine), PCMB, PCMPS, NTCB, and DTNB did not penetrate the intracellular compartment from the luminal media either in control or vasopressin-treated bladders. In contrast, PCMB from the serosal side and NEM from the luminal side titrated significant fractions of the TCA-soluble SH compounds. We conclude, therefore, the PCMB, PCMPS, NTCB, and DTNB are suitable reagents for studies on the physiological properties of apical plasma membrane SH groups. Titration of apical membrane SH groups with PCMPS, NTCB, and DTNB revealed heterogeneity in functional responses: PCMPS and NTCB elicited transient, 25-60% increases in SCC. In substrate-free media, pretreatment with these reagents inhibited the increase in SCC produced by vasopressin or cyclic AMP (+ theophylline). In glucose-enriched media, the responses to combinations of vasopressin and PCMPS or NTCB were additive, implying activation via parallel pathways. Simultaneous addition of vasopressin or cyclic AMP (+ theophylline) and NTCB resulted in marked synergism, presumably as a result of unmasking of SH groups by the the hormone (or the intermediate). These results suggest that basal Na+ transport is regulated in part by SH compounds in the apical membrane that are distinct, although not necessarily different in kind, from those involved in the response to vasopressin.

Although numerous experimental data have been accumulated in the various fields of research on bioelectricity, the mechanism of nerve excitability is still an unsolved problem. Many mechanistic interpretations of nerve behavior cover only a part of the facts, are thus selective and unsatisfactory. An attempt at an integral interpretation of basic data well-established by electrophysiological, biochemical, and biophysical investigations was inspired by the late Aharon Katchalsky and a first attempt had been made previously (Neumann et al., 1973). The present account is a further step toward a quantitative physiochemical theory of bioelectricity. We have further explored the previously introduced notion of a basic excitation unit in excitable membranes. This notion is of fundamental importance for modeling details of sub- and suprathreshold responses, such as threshold behavior and strength-duration curves, in terms of kinetic parameters for specific membrane processes. Our integral model of excitability is based on the original chemical hypothesis for the control of bioelectricity (Nachmansohn, 1959, 1971b). This specific approach includes some frequently ignored experimental facts on acetylcholine-processing proteins in excitable membranes. According to the integral model, acetylcholine ions are continuously processed through the basic excitation units within excitable membranes: axonal, presynaptic, and postsynaptic parts. Excitability, i.e., the generation and propagation of nerve impulses, is due to a cooperative increase in the rate of AcCh translocation through the cholinergic control system.