The plasma membrane is the interface between cells and their harsh environment. Uptake of nutrients and all communication among cells and between cells and their environment occurs through this interface. 'Endocytosis' encompasses several diverse mechanisms by which cells internalize macromolecules and particles into transport vesicles derived from the plasma membrane. It controls entry into the cell and has a crucial role in development, the immune response, neurotransmission, intercellular communication, signal transduction, and cellular and organismal homeostasis. As the complexity of molecular interactions governing endocytosis are revealed, it has become increasingly clear that it is tightly coordinated and coupled with overall cell physiology and thus, must be viewed in a broader context than simple vesicular trafficking.

Regulated portals of entry into the cell

Molecular basis of bacterial outer membrane permeability.

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis an...

http://www.afboard.com/library/creatinemetabolism.pdf

Creatine and Creatinine Metabolism

Production of large unilamellar vesicles by a rapid extrusion procedure. Characterization of size distribution, trapped volume and ability to maintain a membrane potential

Bacteriocins are bacterially produced antimicrobial peptides with narrow or broad host ranges. Many bacteriocins are produced by food-grade lactic acid bacteria, a phenomenon which offers food scientists the possibility of directing or preventing the development of specific bacterial species in food. This can be particularly useful in preservation or food safety applications, but also has implications for the development of desirable flora in fermented food. In this sense, bacteriocins can be used to confer a rudimentary form of innate immunity to foodstuffs, helping processors extend their control over the food flora long after manufacture.

Bacteriocins: developing innate immunity for food

Phosphorus nuclear magnetic resonance was employed to measure the internal pH of unsonicated multilamellar liposomes. A steady transmembrane pH gradient could be obtained in the presence of sulfate and citrate anions. The effects of some uncouplers and ionophores on this controlled pH gradient were studied.

Use of31P NMR to measure pH of membrane-enclosed solutions

During the last years much attention has been focussed on elucidating the process of protein transport in E. coli cells. Proteins synthesized in the cytoplasm but destined for the periplasmic space or outer membrane have to cross the plasma membrane. This process can be divided into several steps; efficient targeting to the membrane, translocation across this membrane and processing into the mature form by cleavage of an N-terminal signal sequence. Genetic approaches, biochemical experiments and more recently the use of reconstituted systems of purified precursor proteins and components involved in protein translocation greatly contributed to an understanding of the early events of bacterial protein export. The results of some studies strongly suggest that cytosolic factors act as chaperone proteins and play an important role in stabilizing a translocation competent conformation of precursors (Crooke et al., 1988; Bochkareva et al., 1989; Collier et al., 1988; Gannon et al., 1988; Kusters et al., 1989; Lecker et al., 1989). The presence of SecA protein, a peripheral protein of the E. coli inner membrane (Oliver & Beckwith, 1982; Schmidt et al., 1988) is essential for a functional binding of such a complex (Cunningham et al., 1989). In addition, SecA protein couples ATP hydrolysis with precursor protein translocation (Lill et al., 1989). Translocation was earlier shown to be an energy dependent process, which requires ATP hydrolysis (Chen & Tai, 1985) and the presence of an electrochemical membrane potential (Geller et al., 1986; De Vrije et al., 1987). Essential for protein translocation across the inner membrane is the presence of a temporal N-terminal extension of the polypeptide chain: the signal sequence. Analogous polypeptide structures are necessary for protein translocation across endoplasmic reticulum membranes in the secretion pathway of eukaryotes. Although there is almost a complete lack in homology of the primary structure of signal peptides, they contain three characteristic regions: a positively charged N-terminal part, a hydrophobic core of 10–15 amino acids and a more polar C-region, containing the cleavage site (Von Heijne, 1985). Figure 1 shows the primary structure of such a signal peptide.

Signal Peptide-Lipid Interactions and their Significance for Protein Translocation in Escherichia coli

Lipids in isolated form show polymorphism. Depending on the type of lipid and experimental conditions, different macromolecular aggregate structures are formed. for membrane lipids hydrated in excess water, under physiological conditions of ionic strength, pH and temperature, three major liquid-crystalline phases are observed. These are: the lamellar phase, the inverted (type II) hexagonal phase (Fig. 1), and the normal (type I) micellar phase. The shape-structure concept of lipid polymorphism as originally proposed by Israelachvili et al. (1976) and adapted by Cullis and De Kruijff (1979) explains remarkably well most of the current data on lipid polymorphism (Fig. 2). If the surface area in the head group region matches that of the hydrocarbon region (such as in phosphatidylcholines) the molecule has an overall cylindrical shape and prefers to organize in extended bilayers. When the head group area exceeds the hydrocarbon area, as for instance in case of detergent-like lipids such as lysophosphatidylcholines, the molecule on average is cone shaped and prefers an organization with a convex (type I) surface curvature such as found in micelles or in the related normal (type I) hexagonal phase occurring for these lipids at low water content. Phosphatidylethanolamines are key representatives of the opposite case of an excess hydrocarbon area. This results in the concave (type II) surface curvature as found in the HII phase. Head group hydration and inter- and intramolecular interactions are important determinants for the shape of the lipid molecules in the shape-structure concept. The strictiy regulated (Goldfine et al., 1987; Wieslander et al., 1981) presence of large amounts of type II lipids (lipids which in hydration prefer to organize in type II structures) and their abundance in virtually all biomembranes suggests important structural and functional roles for these types of lipids.

Functional and Structural Aspects of Gramicidin-Lipid Interactions

Competition between verapamil and doxorubicin for binding to anionic phospholipids: consequences for internal doxorubicin concentration

The lipid matrix of a biomembrane is built with a complex mixture of lipids showing defined variations in polar headgroups and paraffin chains. For membranes with different functions different combinations of lipid species are used. The significance of this lipid diversity and specificity is not yet well understood. It is the aim of this contribution to discuss properties of individual lipid molecules and to show how these properties work out in the barrier functions of membrane model systems. An important aspect will be to see how these barrier functions can be modified by environmental conditions and by intrinsic protein elements. As a consequence of the combination of a polar and an apolar part in the same molecule membrane lipids spontaneously form aggregates when they are dispersed in water. The molecular organization of such aggregates is dependent on the molecular shape of the constituting molecules. Lipids which have a good balance between the size of the polar group and the size of the hydrophobic part easily form bilayer structures (e.g. phosphatidylcholines, phosphatidylglycerols, sphingomyelins). This bilayer organization is in accordance with the lipid organization in the generally accepted molecular models of biomembranes 1. However, it is relevant to emphasize that in each biological membrane also large fractions of lipids occur which in pure form in water adopt other types of molecular arrangements. Figure 1 illustrates that lipid molecules with a polar head of limited size and hydration (e.g. unsaturated phosphatidylethanolamines, monoglucosyldiglycerides, cardiolipin in the presence of Ca2+) prefer an organization of hexagonal packed cylinders, in which the polar heads of the molecules are directed to the inside of the cylinders. Lipid molecules in which the polar headgroups are dominating (e.g. lysolecithins, gangliosides) form a reversed hexagonal (HI) organization upon hydration with a limited amount of water, which is converted into a system of globular micelles in excess of water.

B. de Kruijff

Papers

Use of31P NMR to measure pH of membrane-enclosed solutions

Signal Peptide-Lipid Interactions and their Significance for Protein Translocation in Escherichia coli

Functional and Structural Aspects of Gramicidin-Lipid Interactions

Competition between verapamil and doxorubicin for binding to anionic phospholipids: consequences for internal doxorubicin concentration

Lateral and Polymorphic Phase Transitions in Relation to the Barrier Function of a Lipid Membrane