No abstract

A number of plasma membrane glycoproteins of mammalian and protozoan origin are released from cells by phosphatidylinositol‐specific phospholipase C. Some of these proteins have been shown to be attached to the lipid bilayer via a covalently linked, structurally complex glycophospholipid. Here we establish the existence of similarly linked glycoproteins in the yeast Saccharomyces cerevisiae. The most abundant of these is a tightly membrane‐bound glycoprotein of 125 kd. The detergent‐binding moiety of this protein can be removed by phosphatidylinositol‐specific phospholipase C of bacterial origin or from Trypanosoma brucei. Metabolic labeling indicates that the protein contains covalently attached fatty acid and inositol. It also contains the cross‐reacting determinant (CRD), an antigen found previously on the glycophospholipid anchor of protozoan and mammalian origin. Treatment of the protein with endoglycosidases F and H results in a 95‐kd species. In the secretion mutant sec18, grown at 37 degrees C, the vesicular transport of glycoproteins is reversibly blocked between the rough endoplasmic reticulum and the Golgi apparatus. We find that sec18 cells, when grown at 37 degrees C, do add phospholipid anchors to newly synthesized glycoproteins. This indicates that these anchors are added in the rough endoplasmic reticulum.

The presence of the endogenous Leishmania RNA virus 1 (LRV1) replicating stably within some parasite species has been associated with the development of more severe forms of leishmaniasis and relapses after drug treatment in humans. Here, we show that the disease-exacerbatory role of LRV1 relies on type I IFN (type I IFNs) production by macrophages and signaling in vivo. Moreover, infecting mice with the LRV1-cured Leishmania guyanensis (LgyLRV1 − ) strain of parasites followed by type I IFN treatment increased lesion size and parasite burden, quantitatively reproducing the LRV1-bearing (LgyLRV1 + ) infection phenotype. This finding suggested the possibility that exogenous viral infections could likewise increase pathogenicity, which was tested by coinfecting mice with L. guyanensis and lymphocytic choriomeningitis virus (LCMV), or the sand fly-transmitted arbovirus Toscana virus (TOSV). The type I IFN antiviral response increased the pathology of L. guyanensis infection, accompanied by down-regulation of the IFN-γ receptor normally required for antileishmanial control. Further, LCMV coinfection of IFN-γ-deficient mice promoted parasite dissemination to secondary sites, reproducing the LgyLRV1 + metastatic phenotype. Remarkably, LCMV coinfection of mice that had healed from L. guyanensis infection induced reactivation of disease pathology, overriding the protective adaptive immune response. Our findings establish that type I IFN-dependent responses, arising from endogenous viral elements (dsRNA/LRV1), or exogenous coinfection with IFN-inducing viruses, are able to synergize with New World Leishmania parasites in both primary and relapse infections. Thus, viral infections likely represent a significant risk factor along with parasite and host factors, thereby contributing to the pathological spectrum of human leishmaniasis.Leishmania RNA virus 1 | Totiviridae | arboviruses | trypanosomatid protozoan parasite | Leishmania subgenus Viannia

Numerous glycoproteins of Saccharomyces cerevisiae are anchored in the lipid bilayer by a glycophosphatidylinositol (GPI) anchor. Mild alkaline hydrolysis reveals that the lipid components of these anchors are heterogeneous in that both base‐sensitive and base‐resistant lipid moieties can be found on most proteins. The relative abundance of base‐resistant lipid moieties is different for different proteins. Strong alkaline or acid hydrolysis of the mild base‐resistant lipid component liberates C18‐phytosphingosine indicating the presence of a ceramide. Two lines of evidence suggest that proteins are first attached to a base‐sensitive GPI anchor, the lipid moiety of which subsequently gets exchanged for a base‐resistant ceramide: (i) an early glycolipid intermediate of GPI biosynthesis only contains base‐sensitive lipid moieties; (ii) after a pulse with [3H]myo‐inositol the relative abundance of base‐sensitive GPI anchors decreases significantly during chase. This decrease does not take place if GPI‐anchored proteins are retained in the ER.

Abstract. Saccharomyces cerevisiae contains several abundant phosphoinositol-containing sphingolipids, namely inositolphosphoceramides (IPCs), mannosylinositolphosphoceramide (MIPC), which is substituted on the headgroup with an additional mannose, and M(IP)2C, a ceramide substituted with one mannose and two phosphoinositol groups. Using well-defined temperature-sensitive secretion mutants we demonstrate that the biosynthesis of MIPC, M(IP)2C, and a subclass if IPCs is dependent on genes that are required for the vesicular transport of proteins from the ER to the Golgi. Synthesis of these lipids in intact cells is dependent on metabolic energy. A likely but tentative interpretation of the data is that the biosynthesis of these sphingolipids is restricted to the Golgi apparatus, and that one or more substrates for the biosynthesis of these sphingolipids (phosphatidylinositol, IPCs, or MIPC) are delivered to the Golgi apparatus by an obligatory vesicular transport step. Alternative models to explain the data are also discussed.