Frank Lafont

Bacterial invasion via lipid rafts

Accumulating reports document the use by pathogens of cholesterol‐enriched lipid microdomains, often called lipid rafts, as cell surface platforms to interact, bind and possibly enter into host cells. The challenge is now to understand what could be the functional role of these domains during pathogen invasion. Are they hijacked as general clustering devices for cellular binding sites and/or do they have other roles? In particular, is their cell signalling capacity activated and used by pathogens? In reverse, could lipid rafts activate bacterial mechanisms required for invasion? These issues will be discussed after an introduction on the current view on lipid rafts.

Oiling the key hole

Many bacteria have been found to interact with specialized domains, rich in cholesterol and sphingolipids, of the host plasma membrane, termed lipid rafts. The mechanisms that underlie this interaction are starting to be unravelled. In this issue, Hayward et al. show that early effector proteins secreted by type III secretion harbouring Gram‐negative bacteria are in fact cholesterol‐binding proteins. Combined with other recent findings, this work shows that multiple steps leading to infection by these bacteria depend on raft components: activation of secretion, binding, perforation of the host cell membrane and signalling to trigger bacterial engulfment.

Annexins and their interacting proteins in membrane traffic

SummaryAnnexins are calcium-binding proteins which share common properties due to their homologous core domain. This domain binds phospholipids in a Ca2+-dependent manner. Although extensively studied over 20 years, the function of annexins remains to be elucidated. They are proposed to participate in calcium homeostasis and in the regulation of ion-channel activities, and evidence is accumulating for their role in membrane traffic. Their function is likely to be mediated by their interactions with other proteins such as S100 proteins, C2-domain-containing molecules, and cytoskeletal elements. This review discusses experiments performed in a cellular context, arguing for annexin involvement in exocytosis and endocytosis.

Web alert membranes and sorting membrane permeability

Addresses ‘Department of Cell Biology, Harvard Medical School, 200 Longwood Avenue, Boston, MA 02115, USA; e-mail: kirchhausenQcbr.med.harvard.edu tWellcome/CRC Institute, Tennis Court Road, Cambridge, CB2 1 QR, UK; e-mail: JP103@mole.bio.cam.ac.uk *MERCK KGaA, Bioand Chemolnformatics, 64271 Darmstadt, Germany; e-mail: luca.toldo@merckde %ell Biology Programme, European Molecular Biology Laboratory, Meyerhofstrasse 1, Heidelberg 69012, Germany; e-mail: lafont@EMBL-heidelberg.de.

Web alert Cell-to-cell contact and extracellular matrix

A selection of World Wide Web sites relevant to papers published in this issue of Current Opinion in Cell Biology.

Permeabilized Epithelial Cells to Study Exocytic Membrane Transport

Publisher Summary Polarized exocytic transport in epithelial cells can be measured by performing in vitro assays on filter-grown Madin–Darby canine kidney cells. The transport is assayed either early in the biosynthetic pathway, from the endoplasmic reticulum (ER) to the Golgi complex, or from the trans-Golgi network (TGN) to the apical or from the TGN to the basolateral plasma membrane. Membrane-impermeable molecules can gain access to lipids or membrane proteins facing the cytosol. Each batch of toxin is standardized for the amount of lactate dehydrogenase (LDH) released from the filter-grown MDCK cells, and the amount of LDH released is determined according to a previously described protocol. Measure the protein concentration, which should be around 5 mg/ml. Lower concentrations do not work. When needed, thaw the aliquots quickly and keep them on ice (up to 6 h) until use. Aliquots can be refrozen at least twice. Due to the handling and the various steps, the entire procedure for running one assay requires about 9 h for the apical assay and 11 h for the basolateral assay.

Cell-to-cell contact and extracellular matrix: Web alert

A selection of World Wide Web sites relevant to papers published in this issue of Current Opinion in Cell Biology.

Membrane permeability. Membranes and sorting. Web alert.

A selection of World Wide Web sites relevant to papers published in this issue of Current Opinion in Cell Biology.