The Plasmodium Export Element Revisited

We performed a bioinformatical analysis of protein export elements (PEXEL) in the putative proteome of the malaria parasite Plasmodium falciparum. A protein family-specific conservation of physicochemical residue profiles was found for PEXEL-flanking sequence regions. We demonstrate that the family members can be clustered based on the flanking regions only and display characteristic hydrophobicity patterns. This raises the possibility that the flanking regions may contain additional information for a family-specific role of PEXEL. We further show that signal peptide cleavage results in a positional alignment of PEXEL from both proteins with, and without, a signal peptide

Conserved Domains of the Nullo Protein Required for Cell-Surface Localization and Formation of Adherens Junctions

During cellularization, the Drosophila melanogaster embryo undergoes a transition from syncytial to cellular blastoderm with the de novo generation of a polarized epithelial sheet in the cortex of the embryo. This process couples cytokinesis with the establishment of apical, basal, and lateral membrane domains that are separated by two spatially distinct adherens-type junctions. In nullo mutant embryos, basal junctions fail to form at the onset of cellularization, leading to the failure of cleavage furrow invagination and the generation of multinucleate cells. Nullo is a novel protein that appears to stabilize the initial accumulation of cadherins and catenins as they form a mature basal junction. In this article we characterize a nullo homologue from D. virilis and identify conserved domains of Nullo that are required for basal junction formation. We also demonstrate that Nullo is a myristoylprotein and that the myristate group acts in conjunction with a cluster of basic amino acids to target Nullo to the plasma membrane. The membrane association of Nullo is required in vivo for its role in basal junction formation and for its ability to block apical junction formation when ectopically expressed during late cellularization

Fatty Acid Remodeling of GPI-anchored Proteins Is Required for Their Raft Association

Whereas most of the cellular phosphatidylinositol (PI) contain unsaturated fatty chains and are excluded from rafts, GPI-anchored proteins (APs) unusually contain two saturated fatty chains in their PI moiety, and they are typically found within lipid rafts. However, the origin of the saturated chains and whether they are essential for raft association are unclear. Here, we report that GPI-APs, with two saturated fatty chains, are generated from those bearing an unsaturated chain by fatty acid remodeling that occurs most likely in the Golgi and requires post-GPI-attachment to proteins (PGAP)2 and PGAP3. The surface GPI-APs isolated from the PGAP2 and -3 double-mutant Chinese hamster ovary (CHO) cells had unsaturated chains, such as oleic, arachidonic, and docosatetraenoic acids in the sn-2 position, whereas those from wild-type CHO cells had exclusively stearic acid, a saturated chain, indicating that the sn-2 chain is exchanged to a saturated chain. We then assessed the association of GPI-APs with lipid rafts. Recovery of unremodeled GPI-APs from the double-mutant cells in the detergent-resistant membrane fraction was very low, indicating that GPI-APs become competent to be incorporated into lipid rafts by PGAP3- and PGAP2-mediated fatty acid remodeling. We also show that the remodeling requires the preceding PGAP1-mediated deacylation from inositol of GPI-APs in the endoplasmic reticulum

Lipid Rafts and Pseudotyping

Specific interactions between envelope and core proteins govern the membrane assembly of most enveloped viruses. Despite this, mixed infections lead to pseudotyping, the association of the viral cores of one virus with the envelopes of another. How does this occur? We show here that the detergent-insoluble lipid rafts of the plasma membrane function as a natural meeting point for the transmembrane and core components of a phylogenetically diverse collection of enveloped viruses. As a result, viral particles preferentially incorporate both the envelope components of other viruses as well as the extra- and intracellular constituents of host cell lipid rafts, including gangliosides, glycosyl phosphatidylinositol-anchored surface proteins, and intracellular signal transduction molecules. Pharmacological disruption of lipid rafts interferes with virus production