Team:Freiburg/Content/Project/Receptor

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The AcCELLerator

Murine Cationic Amino Acid Transporter 1 (mCAT-1)

Natural function

Transporters of the cationic amino acid transporter (CAT) family form a class of proteins that occur in mammalian cells as a subfamily of the solute carrier family 7 (SLC7). Expressed nearly ubiquitously in the body, they catalyze the bidirectional transport of cationic amino acids through the cell membrane including the essential amino acids lysine and arginine. [1] In several studies it was shown that this transporter is necessary for basic cell

functions such as protein synthesis, nitric oxide synthesis and inter-organ amino acid flow. Additionally, it plays a key role in recovery after cell stress as it transports essential amino acids into the cell as soon as they become available again. The central importance of the CAT family becomes evident from knockout studies in mice. Deletion of the mouse CAT-1 gene (mCAT-1, SLC7A1 leads to an early death of the animals at the first day after birth.[2]

Structure

In terms of the structure of mCAT-1 it is predicted to exhibit 14 transmembrane domains with intracellular N- and C-terminus. Within the length of 622 amino acids, the third extracellular loop of the receptor is of greatest interest. This site serves as the entry point for the Murine Leukemia Virus (MuLV) and is highly variable between different species. Even close relatives to mice like rats or hamsters exhibit a different CAT-1 that cannot be used by the MuLV as a way in their cells. A reason for this variability of CAT-1 among different species could be a co-evolution of virus (MuLV) and host (mouse). Changes in the part of the mouse genome coding for the third extracellular loop lead to a different structure of the receptor. [4] This change of the viral entry site prohibited viral infection of the mouse forcing the virus to adapt to these changes of CAT-1. As a consequence, the number of hosts of the virus decreased until different mouse species remained. [5]

Fig.1: Scheme of mCAT-1.

mCAT-1 as Viral Entry Side

Found in the year 1989 [6], it was shown that in the presence of mCAT-1 on the surface of mouse cells these cells could be infected by the MuLV. Furthermore, in this publication the receptor of mouse cells was cloned into human cells creating a selective susceptibility to infection by MuLV. Studies of Allbriton et al. have shown that amino acids in the extracellular loop three of mCAT-1 are critical for virus binding.

As a consequence, mCAT-1 is well suited to create a selective entry in cells for the following reasons:

  • As it is highly variable, there are not many viruses that have adapted to this receptor. Accordingly, the risk of other viruses using this receptor as an entry site is very small – there are no known other viruses using mCAT-1 to enter cells à it is safe to work with mCAT-1
  • As it is highly variable, the receptor is not expressed on most of the commonly used cell lines; for example, we used HEK297 (human embryonic kidney) cells as well as CHO-K1 (chinese hamster ovary) cells; viral entry by MuLV was impossible until the expression of mCAT-1 was induced by light induction.
  • As it is known since 1989, there are many publications and protocols that can be used for an appropriate experimental procedure.
  • As it is known since 1989, it is not in possession of any company and can be used for cloning easily.

    • These (and others?) are the reasons why we chose mCAT-1 for our project as a selective surface protein. Since we heard until now a lot about the light system and the receptor mCAT-1, we would like to tell you more about the viral vectors in general. Afterwards, we would like to focus on MuLV in particular, the viral vector that is using mCAT-1 as entry site to deliver its cargo.

    List of Literature

    1. Kozak, C.A., Evolution of different antiviral strategies in wild mouse populations exposed to different gammaretroviruses. Curr Opin Virol, 2013. 3(6): p. 657-63.
    2. Hatzoglou, M., et al., Regulation of cationic amino acid transport: the story of the CAT-1 transporter. Annu Rev Nutr, 2004. 24: p. 377-99.
    3. Closs, E.I., et al., Structure and Function of Cationic Amino Acid Transporters (CATs). The Journal of Membrane Biology, 2006. 213(2): p. 67-77.
    4. Ferrarone, J., et al., Second site mutation in the virus envelope expands the host range of a cytopathic variant of Moloney murine leukemia virus. Virology, 2012. 433(1): p. 7-11.
    5. Kozak, C.A., Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity. Advances in Virology, 2011. 2011: p. 16.
    6. Albritton, L.M., et al., A putative murine ecotropic retrovirus receptor gene encodes a multiple membrane-spanning protein and confers susceptibility to virus infection. Cell, 1989. 57(4): p. 659-666.
    7. Allbritton et al., Envelope-binding domain in the cationic amino acid transporter determines the host range of ecotropic murine retroviruses.1993

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