2011) by biolistic transformation. infection, which is normally controlled by alveolar macrophages. In immunocompromised individuals, however, fungi can proliferate and disseminate throughout the host, with a particular tropism for the brain. The principal virulence factor of is a polysaccharide capsule, which helps it evade the host immune response. This structure surrounds the yeast cell wall, which as in other fungi is made of glucans, chitin, and mannoproteins (Doering 2009). The capsule is composed of two polysaccharides, glucuronoxylomannan (GXM) and glucuronoxylomannogalactan (GXMGal), with trace amounts of mannoproteins (Kumar et al. 2011). These polymers, which are synthesized intracellularly (Yoneda and Doering 2006), become associated with the outer surface of the cryptococcal cell wall (Reese et al. 2007), forming a protective layer that impedes phagocytosis and immune mediator binding (Voelz and May 2010). This structure is highly responsive to environmental conditions, becoming particularly large during infection of mammalian hosts (Doering 2009). Capsule polysaccharides are also continually shed from the yeast, and act in suppression of the host immune response (Voelz and May 2010). Understanding the pathways that produce the capsule and other essential glycoconjugates is central to developing strategies to effectively disrupt their function and combat this lethal pathogen. GXM typically constitutes ~90% Acetaminophen of the capsule mass (Doering 2009). It is a repeating polymer, made up of a mannose backbone with glucuronic acid (GlcA) and xylose (Xyl) side chains (Cherniak et al. 1998) (all sugars are pyranose forms unless otherwise indicated). GXMGal, which makes up the remaining 10% of the capsule mass, consists of a galactan backbone modified with galactomannan side chains bearing a variable number of Xyl and GlcA residues (Heiss et al. 2009); the backbone may also be modified with single galactofuranose (Galf) residues (Heiss et al. 2013). Mutants lacking either or both capsular polysaccharides are avirulent (Chang and Kwon-Chung 1994; Moyrand et al. 2007). dedicates a significant portion of its genetic machinery and metabolic energy to synthesizing capsule and other cellular glycoconjugates, including protein-linked glycans (Olson et al. 2007; Reilly et al. 2011; Park et al. 2012), cell wall components (Reese and Doering 2003; Banks et al. 2005; Reese et al. 2007; Gilbert et al. 2010; Gilbert et al. 2011) and glycolipids (Vincent and Klig 1995; Heise et al. 2002; Rittershaus et al. 2006; Castle et Acetaminophen al. 2008). These compounds are essential for maintaining cellular homeostasis and establishing infection. Synthesis of many glycoconjugates relies on activated donor molecules, such as nucleotide sugars, from which individual sugar moieties are transferred to a growing glycan structure. Nucleotide sugars are generally made in the cytosol and then transported into the secretory organelles (endoplasmic reticulum and/or Golgi apparatus) where most glycan biosynthesis occurs (Varki et al. 2009). Nucleotide sugar transporters (NSTs) mediate transport of these highly charged compounds by importing them in exchange for the corresponding nucleotide monophosphates Acetaminophen via an antiport mechanism (Abeijon et al. 1997; Berninsone and Hirschberg 2000). This makes the nucleotide sugars available for use by the luminal glycosyltransferase enzymes that synthesize capsule polymers or other glycans. The capsule polysaccharides are composed of galactose (Gal), Galf, GlcA, Man and Xyl; this suggests that their synthesis requires the corresponding donors, which are UDP-Gal, UDP-Galf, UDP-GlcA, GDP-Man and UDP-Xyl. The enzymatic pathways required for synthesis of these compounds in have been elucidated (Bar-Peled et al. 2001; Wills et al. 2001; Bar-Peled et al. 2004; Griffith et al. 2004; Beverley et al. 2005; Moyrand et al. 2008), but the identity and the regulation of most of the NSTs that translocate them into the secretory pathway remain elusive. Only transport of the mannose Rabbit Polyclonal to JAK1 donor, GDP-mannose, has been demonstrated biochemically (Cottrell et al. 2007). Strains deficient in UDP-Gal synthesis have aberrant capsule, likely due to perturbed GXMGal production, and are completely avirulent, emphasizing the critical role of this nucleotide sugar (Moyrand et al. 2008). A UDP-Gal transporter, Ugt1, was initially identified by homology to known UDP-Gal transporters (Moyrand et al. 2007), which are found ubiquitously in eukaryotes. However, sequence identity does not tell the whole story of NST substrate specificity (Martinez-Duncker et al. 2003). For example, cells may express several NSTs with overlapping specificities but non-identical substrate affinities. Some transporters are highly selective for a specific substrate, while others transport as many Acetaminophen as four distinct nucleotide sugars (Berninsone et al. 2001; Norambuena et al. 2002; Segawa et al. 2002; Aoki et al. 2003; Ashikov et al. 2005;.
Categories:Retinoid X Receptors