Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.6.3.44 (P-glycoprotein)
 13,344 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Increased expression of P-glycoprotein (Pgp) has been demonstrated to cause multidrug resistance (MDR) in vitro, and it may be responsible for chemotherapy failure in a number of human cancers. Pgp is a plasma membrane protein thought to function as an energy-dependent drug transporter. From its deduced protein sequence the topology of Pgp was proposed to contain 12 transmembrane domains with six extracellular loops and two cytoplasmic ATP-binding sites. To investigate further the membrane orientation of Pgp, we have expressed a full length cDNA of mouse mdr1, as well as its truncated forms, in a cell-free system supplemented with dog pancreatic microsomal membranes (RM). We determined which domains of the in vitro-synthesized Pgp had transversed the RM membranes by analyzing their resistance to protease digestion and their glycosylation state. To our surprise, this system revealed that a significant portion of in vitro-synthesized Pgp molecules has an additional glycosylated domain in the C-terminal half. Previously, only the first predicted extracellular loop near the N terminus had been thought to be glycosylated. Furthermore, we discovered that Pgp has at least two functional signal recognition particle/docking protein dependent signal sequences, one at the N-terminal half and the other at the C-terminal half. These findings suggest a new topological model for in vitro synthesized P-glycoprotein which may be relevant to its in vivo topology.
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PMID:Study of membrane orientation and glycosylated extracellular loops of mouse P-glycoprotein by in vitro translation. 168 Aug 60

Transmembrane topology of polytopic integral membrane proteins is established during protein synthesis at the endoplasmic reticulum membrane. For some polytopic proteins, sequential and independent signal, stop transfer, and/or signal anchor sequences contained in the nascent chain direct this process. Here we define the topology of human P-glycoprotein (MDR1) through the first two transmembrane regions (TM1 and TM2, respectively) of the amino-terminal half of the protein. We show that unlike TM7 and TM8, which comprise homologous regions in the carboxyl half of the protein (Skach, W., Calayag, M. C., and Lingappa, V. (1993) J. Biol. Chem. 268, 6903-6908), TM1 and TM2 achieve the orientation predicted by conventional structural models. However, TM1 and TM2 appear to utilize a mechanism of biogenesis different in a key respect from that observed in multispanning proteins studied previously. TM1 and TM2, with their flanking regions, independently direct the topology observed for each of these sequences in the native protein. Each can interact with signal recognition particle to direct targetting to the endoplasmic reticulum, nascent chain translocation, and correct transmembrane orientation. Unlike the transmembrane regions of previously studied multispanning membrane proteins, neither TM1 nor TM2 alone is sufficient to integrate the chain into the membrane. However, when TM1 and TM2 are both present, as occurs in native MDR1, integration is achieved. These results suggest that cooperative interactions between TM1 and TM2 are necessary for chain integration and thus add a new complexity to the current view of polytopic integral membrane protein biogenesis.
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PMID:Amino-terminal assembly of human P-glycoprotein at the endoplasmic reticulum is directed by cooperative actions of two internal sequences. 790 Dec 9

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated C1(-) channel. Malfunction of CFTR causes cystic fibrosis (CF). CFTR belongs to an ATP-binding cassette (ABC) transporter superfamily which includes P-glycoprotein (Pgp), the molecule that is responsible for multidrug resistance in cancer cells. P-glycoprotein molecules have been suggested to have more than one topology and function. In this study, we analysed the early stages of membrane insertion, processing, and topology of human CFTR using rabbit reticulocyte lysate and wheat germ extract translation systems supplemented with canine pancreatic microsomal membranes. Our results suggest that CFTR contains an uncleavable signal sequence and its membrane targeting and insertion may depend on the signal recognition particle (SRP) and SRP receptor. The topology of CFTR in microsomal membranes is the same as the one predicted based on hydropathy plot analysis. These results, together with our previous findings on Pgp, indicate that (1) the topologies of mammalian ABC transporters can be dissected and studied using protein fusion chimeras in a cell-tree system; and (2) the membrane targeting and insertion of CFTR and Pgp may take the same pathway, i.e., the SRP-dependent pathway, but the membrane folding mechanism of these two proteins in microsomal membranes is probably different.
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PMID:Membrane insertion, processing, and topology of cystic fibrosis transmembrane conductance regulator (CFTR) in microsomal membranes. 914 60