One of the hallmarks of eukaryotic cells is the presence of complex intracellular membrane-bound organelles dedicated to specific functions. Part of the structural and functional specificity of these organelles is based on their distinct complement of proteins and membrane lipids. The link between membrane traffic, protein traffic, and organelle biogenesis is now becoming evident in the context of the secretory pathway (Derby and Gleeson, 2007
). Failure of any component within the pathway can lead to abnormal targeting of proteins and membrane components that are necessary for organelle function or biosynthesis (Howell et al., 2006
). Furthermore, some of these defects in organelle biogenesis have been found to be associated with human diseases (Dhaunsi, 2005
Vacuole membrane protein 1 (Vmp1) is a conserved putative membrane protein with no recognizable functional motifs. The function of Vmp1 is now beginning to be elucidated. Several lines of evidence suggest a possible role of this protein in membrane traffic and organelle organization. It has been described as a stress-induced endoplasmic reticulum (ER) protein in the rat exocrine pancreas that is highly expressed during acute pancreatitis (Dusetti et al., 2002
; Vaccaro et al., 2003
). Overexpression of this protein in cell culture leads to vacuole formation and cell death, a process that is observed in pancreatitis (Dusetti et al., 2002
). A recent report also identified Vmp1 as a novel autophagy-related membrane protein involved in mammalian pancreatitis-induced autophagy (Ropolo et al., 2007
). In Drosophila
, Vmp1 (known as TANGO-5) was also identified in a functional genomic screen by using RNA interference. TANGO-5 was found to be required for protein secretion and Golgi organization (Bard et al., 2006
). In another study, Vmp-1 was localized in the plasma membrane in the kidney cancer cell line Caki-2, and it was found to be essential for cell–cell contact (Sauermann et al., 2008
). These results suggested a totally different function of Vmp1 in tumor cells. Therefore, the function of this protein remains controversial and seems to depend on the specific cell type studied.
Vmp1 is a conserved protein and the study of its complex function might benefit from the use of simple experimental systems such as Dictyostelium discoideum
. Using the completed Dictyostelium
genome sequence, we generated a collection of mutants by targeted disruption of genes with unknown function that are highly conserved between Dictyostelium
and humans, but also absent from the genomes of Saccharomyces cerevisiae
and Schizosaccharomyces pombe
(Torija et al., 2006a
). Among those genes, we identified Vmp1 (named DupF in that analysis). Its absence in any fungi makes Dictyostelium
the simplest genetically tractable model system to address its function.
is a eukaryotic microorganism used as a model to study basic cellular processes, including membrane traffic and the endocytic pathway (Maniak, 2003
). These social amoebae live as solitary cells feeding on other microorganisms by phagocytosis. Laboratory strains are also capable of growth in axenic media that is taken up by macropinocitosis. The vacuoles of ingested material fuse with lysosomes and undigested residues are secreted by exocytosis. As in many soil microorganisms, water regulation is essential for survival. A specialized organelle, the contractile vacuole (CV) system, is composed of an independent network of membrane tubules and cisternae that fill up and expel water by transient fusion with the plasma membrane (Gabriel et al., 1999
). Contractile vacuole biogenesis is dependent on clathrin-coated vesicles and the adaptor-protein complex 1 (AP-1) for transporting protein and membranes required for the CV formation (O'Halloran and Anderson, 1992
; Lefkir et al., 2003
). As a result, defects in AP-1 function lead to impaired osmoregulation.
Besides its interest as a cellular model, Dictyostelium
has the exceptional ability to form a multicellular organism by aggregation of solitary cells. The differentiation program is triggered by starvation and leads to the formation of a fruiting body composed of spores supported by a stalk (Escalante and Vicente, 2000
In this report, we describe the first loss-of-function mutation for a Vmp1 homologue in a model system. We have found that Vmp1 is an endoplasmic reticulum protein in Dictyostelium necessary for the integrity of this organelle. The lack of this ER protein has pleiotropic defects in several membrane traffic-dependent processes such as organelle biogenesis and structure, endocytosis, and protein trafficking. Our results also suggest that an aberrant pattern of protein secretion during starvation might in part account for the impairment in the transition from growth to development in Dictyostelium.