Membrane Proteins
Cell Membrane

Author: Gianpiero Pescarmona
Date: 03/08/2008

Description

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Topology of membrane proteins

Intrinsic Proteins

A transmembrane protein is a protein that spans the entire biological membrane. Transmembrane proteins aggregate and precipitate in water. They require detergents or nonpolar solvents for extraction, although some of them (beta-barrels) can be also extracted using denaturing agents.

Membrane Protein Inserption

GPI-anchored Proteins

  • The relationship between the structures and functions of the GPI anchor is difficult to study due to the lack of sufficient quantities of pure anchors and anchored proteins. When produced in cells, GPI-anchored proteins exist as heterogeneous mixtures with considerable variation in their glycan core modifications and lipid moieties, a complicating feature with respect to functional analysis (1, 13, 18, 58). Furthermore, well-defined modifications to the GPI anchor structure cannot be imposed using conventional biological methods; the biosynthetic enzymes are not well characterized, and their disruption in cells simply leads to loss of the entire GPI structure.
    The GPI-anchor and protein sorting 2001

  • The GPI anchor is a structurally complex posttranslational modification that remains a mystery with respect to its biological activities.
  • Protein Moiety
    • PrP

Table 1. Functional diversity of GPI-anchored proteins.

  • Enzymes Surface antigens
    • Acetylcholinesterase Thy-1, Ly-6 (TAP), Qa-2,
    • alkalinephosphatase, 5¢- Sca-2, CD24, CD48 (sgp-60),
    • nucleotidase, dipeptidase, CD52 (CAMPATH-1), CD-5
    • lipoprotein lipase, ART1 (DAF), CD59, CD73,
    • ART2 (RT6) cerebroglycan,
  • Adhesion molecules
    • ceruloplasmin, prion proteins
    • NCAM, ApCAM, OBCAM Other (lower eukaryotes)
    • F3/F11/contain), TAG-
  • Variant surface glycoprotein
    • 1 (axonin-1), fasciclin II, (VSG), SSp-4, sialic acid
    • BIG-1, BIG-2, neurotrimin, acceptor, 160-kDa flagellar
    • LFA-3 antigen, CS protein, MSP-1
  • Receptors
    • protease, MSP-2, MSP-4,
    • Folate receptor, CNTFR-a gp63 metalloprotease, PSA-2,
    • GDNFR-a, CD87 (uPAR) PARP/procyclin,
    • NTNR-a, CD14, glypicans GP-2
      FcgRIIIb,

The Glycosylphosphatidylinositol Anchor: A Complex Membrane-Anchoring Structure for Proteins 2008

Table 2: Representative Functions of GPI-Anchored Proteins (2, 3, 17, 27)
biological role

protein

source
enzymes

alkaline phosphatase

mammalian tissues, Schistosoma


5′-nucleotidase

mammalian tissues


acetylcholinesterase

Torpedo electric organ, insect brain, mammalian blood cells


dipeptidase

pig and human kidney, sheep lung
cell−cell interaction

LFA-3

human blood cells


NCAM

mammalian and chicken brain and muscle


PH-20

guinea pig sperm
complement regulation

CD55 (DAF)

human blood cells


CD59

human blood cells
mammalian antigens

Thy-1

mammalian brain and lymphocytes


Qa-2

mouse lymphocytes


CD14

human monocytes


carcinoembryonic antigen (CEA)

human tumor cells


CD52

human lymphocytes
protozoan antigens

VSG

T. brucei


1G7

T. cruzi


procyclin

T. brucei
miscellaneous

scrapie prion protein

hamster brain


CD16b

human neutrophils


folate-binding protein

human epithelial cells

Differential sorting and fate of endocytosed GPI-anchored proteins 2002

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