Team:Freiburg/Project/Receptor

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The AcCELLerator

The AcCELLerator

The Receptor - 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 interorgan amino acid transport. 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 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

The mCAT-1 has 14 putative transmembrane segments with intracellular N- and C-termini. Within the length of 622 amino acids, the third extracellular loop of the receptor is most interesting. 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 an entry point. 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 region 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. Members of the CAT family are predicted to have 14 transmembrane domains with intracellular N- and C-termini. Two asparagine residues in the third extracellular loop (indicated as branched lines) have been shown to be glycosylated [8].

mCAT-1 as Viral Entry Site

The mouse CAT-1 was originally identified by Albritton in 1989 as the receptor for murine ecotropic leukemia viruses (MuLV) [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. However, human cells acquire the susceptibility to infection by MuLV only if the cells express mCAT-1 ectopically. Studies of Albritton et al. have shown that amino acids in the extracellular loop three of mCAT-1 are critical for virus binding [7].

As a consequence, mCAT-1 is well suited to create a selective entry into 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 → the work with mCAT-1 is categorized as Biosafety Level 1,
  • the receptor is not expressed on most of the commonly used cell lines; for example, HEK-293T (human embryonic kidney) cells as well as CHO-K1 (chinese hamster ovary) cells can be utilized.

Because the virus is known since 1989,

  • there are many publications and protocols that can be used for an appropriate experimental procedure,
  • it is not in possession of any company and can be used for cloning easily.

These are the reasons why we chose mCAT-1 for our project as a selective surface protein. Knowing 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. We will also focus on the MuLV in particular, the viral vector that uses mCAT-1 as entry site to deliver its cargo.
Read More about viral vectors

References

  1. Kozak CA (2013). Evolution of different antiviral strategies in wild mouse populations exposed to different gammaretroviruses. Curr Opin Virol 3:657-663.
  2. Hatzoglou M, Fernandez J, Yaman I, Closs E (2004). Regulation of cationic amino acid transport: the story of the CAT-1 transporter. Annu Rev Nutr 24:377-399.
  3. Closs EI, Boissel JP, Habermeier A, Rotmann A (2006). Structure and Function of Cationic Amino Acid Transporters (CATs). J Membr Biol 213:67-77.
  4. Ferrarone J, Knoper RC, Li R, Kozak CA (2012). Second site mutation in the virus envelope expands the host range of a cytopathic variant of Moloney murine leukemia virus. Virology 433:7-11.
  5. Kozak CA (2011). Naturally Occurring Polymorphisms of the Mouse Gammaretrovirus Receptors CAT-1 and XPR1 Alter Virus Tropism and Pathogenicity. Adv Virol 2011, Article ID 975801.
  6. Albritton LM, Tseng L, Scadden D, Cunningham JM (1989). A putative murine ecotropic retrovirus receptor gene encodes a multiple membrane-spanning protein and confers susceptibility to virus infection. Cell 57:659-666.
  7. Albritton LM, Kim JW, Tseng L, Cunningham JM (1993). Envelope-binding domain in the cationic amino acid transporter determines the host range of ecotropic murine retroviruses. J Virol 67:2091-2096.
  8. Louis J. Ignarro, Nitric Oxide: Biology and Pathobiology (2009).

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