Major histocompatibility complex (MHC) class I molecules are ligands for T-cell receptors of CD8؉ T cells and inhibitory S1A), using the crystal structure of the closely related protein, calnexin (2). Calreticulin is thought to contain three structural domains. The first is a large globular domain comprising the Nand C-terminal regions of the protein that form a ␤-stranded sandwich and a C-terminal helix (supplemental Fig. S1A, orange, light blue, and green). The glycan binding site is located within this domain (supplemental Fig. S1B). A second hookshaped P-domain forms a ␤-stranded hairpin structure inserted in the middle of the globular domain (supplemental Fig. S1A, dark blue). The tip of this domain forms a binding site for ERp57 (3), an ER oxidoreductase that works cooperatively with calreticulin and calnexin in glycoprotein folding (4). A third C-terminal domain, rich in acidic amino acids, is functional as a low affinity/high capacity calcium coordination site (1). This region is not present in the lumenal domain of calnexin.In vitro studies have shown that calreticulin can bind to misfolded non-glycosylated polypeptides and suppress their irreversible aggregation (5). This activity is induced by various conditions associated with ER stress, including calcium depletion and heat shock (6). These conditions also induce calreticulin oligomerization (6, 7). Much remains to be understood about the one or more binding sites on calreticulin that are used to suppress substrate aggregation, as well as the relevance of this activity to calreticulin-mediated protein folding under physiological non-stress conditions.Calreticulin is a key player in the MHC class I assembly pathway (8). The MHC class I-dedicated assembly factors, transporter associated with antigen processing (TAP) and tapasin, as well as the generic ER-folding factors ERp57 and calreticulin, form a large complex with MHC class I molecules, collectively called the PLC. TAP provides a major source of peptides for MHC class I molecules, whereas tapasin, ERp57, and calreticulin facilitate assembly of MHC class I molecules with peptides (reviewed in Ref. 9). Calreticulin is a component of the PLC, and calreticulin-deficient cells express reduced cell surface MHC class I molecules (8). The mechanisms by which calreticulin contributes to enhanced MHC class I assembly are not well understood.Early studies with glycosylation inhibitors, MHC class I mutants, and in vitro binding analyses suggested that glycanbased interactions with MHC class I molecules recruit calreti-* This work was supported, in whole or in part, by National Institutes of Health Grant AI066131 (to M. R.) and by a diversity supplement to National Institutes of Health Grant AI066131 (awarded to N. D.). □ S The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S3 and

Valacyclovir is the 5-valyl ester prodrug of acyclovir, an effective anti-herpetic drug. Systemic availability of acyclovir in humans is three to five times higher when administered orally as the prodrug. The increased bioavailability of valacyclovir is attributed to carrier-mediated intestinal absorption, via the hPEPT1 peptide transporter, followed by the rapid and complete conversion to acyclovir. The one or more human enzymes responsible for in vivo activation of the prodrug to the active drug and its conversion sites, however, have not been identified. In this report, we describe the purification, identification, and characterization of a human enzyme that activates valacyclovir to acyclovir. A protein with significant hydrolytic activity toward valacyclovir, the 5-glycyl ester of acyclovir, and the 5-valyl ester of zidovudine (AZT), was purified from Caco-2 cells derived from human intestine. Using a non-redundant data base search, the N-terminal 19-amino acid sequence of the purified 27-kDa, basic protein revealed a perfect match within the N terminus of a serine hydrolase, Biphenyl hydrolase-like (BPHL, gi:4757862) protein, previously cloned from human breast carcinoma. Recombinant BPHL exhibited significant hydrolytic activity for both valacyclovir and valganciclovir with specificity constants (k cat /K m ), 420 and 53.2 mM ؊1 ⅐s ؊1 , respectively. We conclude that BPHL may be an important enzyme activating valacyclovir and valganciclovir in humans and an important new target for prodrug design.Prodrugs of therapeutically active agents have been used to improve pharmaceutical, biopharmaceutical, and pharmacokinetic properties of numerous active therapeutic agents. Prodrugs are designed to be inactive until in vivo activation to the parent drug, and hence reliable in vivo activation of the prodrug is considered critical for their pharmacological activity (1). Identification of the mechanism of in vivo activation of prodrugs is important for prodrug design and for investigating clinical applications. Furthermore, design and development of prodrugs for humans has been significantly hampered by the unknown species differences in the activating enzymes. Thus, identification of the one or more prodrug-activating enzymes will significantly aid in the selection of animal models for human drug development.The participation of peptidases or esterases in prodrug activation will depend on the pro-moiety, its linker to the parent drug, as well as the parent drug. For several prodrugs, their in vivo activation mechanism has been studied in more detail. For example, the anti-cancer prodrug, CPT-11 (irinotecan), a carbamate derivative of 7-ethyl-10-hydroxycamptothecin, is converted to its active metabolite, 7-ethyl-10-hydroxycamptothecin by human carboxylesterases. The efficiency of hydrolysis varies depending on isoforms such that carboxylesterase 2 (hCE2) 1 and intestinal carboxylesterase (hiCE) are more efficient activators than human liver carboxylesterase 1 (hCE1) (2-4). The angiotensin-converting enzyme inhibitor...

Gelonin is a type I plant toxin that has potential as an effective anti-tumor agent by virtue of its enzymatic capacity to inactivate ribosomes and arrest protein synthesis, thereby effectively limiting the growth of cancer cells. Being a hydrophilic macromolecule, however, gelonin has limited access to its target subcellular compartment, the cytosol; it is effectively plasma membrane-impermeant and subject to rapid degradation within endosomes and lysosomes upon cellular uptake as it lacks the membrane-translocating capability that is typically provided by a disulfide-linked B polypeptide found in the type II toxins (e.g. ricin). These inherent characteristics generate the need for the development of a specialized cytosolic delivery strategy for gelonin as an effective anti-tumor therapeutic agent. Here we describe an efficient means of delivering gelonin to the cytosol of B16 melanoma cells. Gelonin was co-encapsulated inside pH-sensitive liposomes with listeriolysin O, the pore-forming protein that mediates escape of the intracellular pathogen Listeria monocytogenes from the endosome into the cytosol. In in vitro experiments, coencapsulated listeriolysin O enabled liposomal geloninmediated B16 cell killing with a gelonin IC 50 of ϳ0.1 nM with an extreme efficiency requiring an incubation time of only 1 h. By contrast, cells treated with equivalent concentrations of unencapsulated gelonin or gelonin encapsulated alone in pH-sensitive liposomes exhibited no detectable cytotoxicity. Moreover, treatment by direct intratumor injection into subcutaneous solid tumors of B16 melanoma in a mouse model showed that pH-sensitive liposomes containing both listeriolysin O and gelonin were more effective than control formulations in curtailing tumor growth rates.Delivery of exogenous macromolecules to the cytosol is a fundamentally inefficient process. This difficulty arises from the fact that cells have an obligation to maintaining homeostasis; hence the need for strict control over what is allowed passage, intact, into the cell. Because of their hydrophilicity and large hydrodynamic volumes, macromolecules such as DNA and protein are effectively impermeant to the cell's plasma membrane. Those that are taken up by cells via, for example, fluid-phase or receptor-mediated endocytosis are ultimately degraded within the endosomal/lysosomal pathway, or in some cases are returned to the extracellular environment (1, 2). The ultimate fate of internalized macromolecules may not be of concern where the site of action is at the cell surface (e.g. insulin). For many biomolecules with therapeutic potential, however, direct interaction with an intracellular target may be a prerequisite for efficacy. This condition is particularly true for many plant-derived toxins that have cytostatic or cytotoxic activities and thus have potential as anti-cancer therapies.Plant toxins currently used or envisioned in pharmaceutical formulations are predominantly either type I or type II toxins. The type II toxins are composed of two disulfide-linked po...

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