Plasma membranes general review

Early work on spermatogenesis has been reviewed in the first edition (796). More recent work has been reviewed by Euzet et al. (196), Davis Roberts (171), Ubelaker (889) and Lumsden Specian (462). Ultrastructure studies have shown that cestode spermatozoa generally (but see below) have the same pattern of organisation in all orders. The chief features are (a) a long thread-like body (b) an elongated nucleus (c) cortical microtubules underlying the plasma membrane (d) the absence of... 

The latter effects were inhibited by pretreatment with antibodies to PLC-P3 but not with antibodies to other PLC-P isoezymes. They were also diminished by antibodies to the G protein P subunit but not to G protein tti and ao subunits. These results are in line with the observation that efforts to identify PTX-sensitive G protein a subunits that activate phos-phatidylinositol-phospholipase C isoenzymes have generally been unsuccessful. It is now considered that stimulation of phosphatidylinositol biphosphate hydrolysis by the PTX-sensitive G proteins, Gj and Go, involves their Py subunits (Exton 1996). Inhibition of cAMP formation was partially blocked by Gan and Gtto antibodies and additively blocked by a combination of both antibodies. SST-induced S-GTPyS-Ga complexes in plasma membranes were bound selectively by Gaii and GOo antibodies (Murthy et al. 1996). The authors concluded from these results that in smooth muscle sst3 is coupled to Gu and Go. Their a subunits mediate the inhibition of AC, while the Py subunits mediate activation of PLC-P3. Both Gai and Gao subunits are known to be moderately effective inhibitors of Ca " " - calmodulin-stimulated AC type I. Gat, in addition, also inhibits type V and VI AC (for review see Sunahara et al. 1996). 

The purpose of this review is to present the current status of one such system, the glucagon-sensitive adenyl cyclase from liver plasma membranes. Information obtained from other adenyl cyclase systems will be presented only to expand the general significance of the liver system. 

In a review on membrane proteins, Guidotti (1972) has classified membranes into three types on the basis of their protein content. The first class is the simple, inert membrane represented by myelin. It consists primarily of lipid with little protein, acts as a permeability barrier and insulator, and has only three known enzymatic activities (Beck et al., 1968 Olafson et al., 1969 Kurihara and Tsukada, 1967 Gammer et al., 1976 Yandrasitz et al., 1976). The large second class of membranes which have a protein-to-lipid ratio of about 1 1 (w w) are typified by most mammalian plasma membranes. They have many enzymatic activities and sophisticated transport systems associated with them, in addition to the permeability factor. The third class of membranes has bacterial and inner mitochondrial membranes as its models. These membranes have proportionately larger amounts of protein than lipid and have added functions such as oxidative phosphorylation and nucleic acid synthesis. In general, the specialization and enzyme function of the membrane increases in proportion to its protein content. Table 4 gives the amino acid composition of some isolated membrane proteins. Total membrane protein (intrinsic + extrinsic) often has an amino acid composition which falls into the range of other nonmembrane, "soluble" proteins (Vanderkooi and Capaldi, 1972). 

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