1.C.6 The Yeast Killer Toxin K1 (YKT-K1) Family

A single, well-characterized yeast killer toxin, K1, produced by Saccharomyces cerevisiae killer strains that bear the specific satellite dsRNA which encodes the killer toxin, comprises the YKT-K1 family. A strain that produces the toxin is resistant to it, but other yeast strains are sensitive. The toxin is made as a preprotoxin which is processed during secretion via the endoplasmic reticulum, Golgi apparatus and secretory vesicles to yield the mature, active extracellular toxin.

K1 (19 kDa) is the best studied yeast killer toxin. It consists of two distinct, disulfide-bonded, unglycosylated subunits, α(9.5 kDa) and β (9.0 kDa) derived from a glycosylated 42 kDa protoxin. The protoxin is derived from the dsRNA-encoded, 316 amino acyl, 35 kDa preprotoxin M1p. The preprotoxin consists of an N-terminal 44 amino acyl leader sequence (δ), including a 26 amino acyl signal peptide, the 103 residue α-domain (positions 45-147), an 85 residue γ-peptide and the 83 residue β-domain (positions 234-316). The α- and β-subunits are disulfide-bonded in the mature toxin.

K1 kills susceptible yeast cells by (1) binding to a cell wall receptor, (2) binding to a membrane receptor, (3) inserting into the cytoplasmic membrane and (4) forming a voltage-independent cation-selective transmembrane channel which causes ion leakage and subsequent cell death. Two strongly hydrophobic regions near the C-terminus of the α-subunit (residues 72-91 and 112-130) form α-helical structures that presumably insert into the membrane to form the oligomeric channel. Some other nonhomologous yeast killer toxins are believed to function as channels, but many others kill target yeast cells by unrelated mechanisms.

K1 toxin is encoded by a yeast viral gene and targets virus-free cells. K1 is known to produce uncontrolled leakage of K+, H+, ATP, and other small molecules from the target cell. Recently it has been suggested that the major killing effect is due to the loss of K+, and that the toxin functions in this capacity by activating the yeast TOK1 K+ channel rather than (or in addition to) forming a transmembrane channel (Ahmed et al., 1999).

The generalized transport reaction catalyzed by yeast killer toxin K1 is:

ions (in) ions (out).



This family belongs to the .

 

References:

Ahmed, A., F. Sesti, N. Ilan, T.M. Shih, S.L. Sturley, and S.A.N. Goldstein. (1999). A molecular target for viral killer toxin: TOK1 potassium channels. Cell 99: 283-291.

Becker, B. and M.J. Schmitt. (2017). Yeast Killer Toxin K28: Biology and Unique Strategy of Host Cell Intoxication and Killing. Toxins (Basel) 9:.

Belda, I., J. Ruiz, A. Alonso, D. Marquina, and A. Santos. (2017). The Biology of Pichia membranifaciens Killer Toxins. Toxins (Basel) 9:.

de la Pena, P., F. Barros, S. Gascon, P.S. Lazo, and S. Ramos. (1981). The effect of yeast killer toxin on sensitive cells of the Saccharomyces cerevisiae. J. Biol. Chem. 256: 10420-10425.

Magliani, W., S. Conti, M. Gerloni, D. Bertolotti, and L. Polonelli. (1997). Yeast killer systems. Clin. Microbiol. Rev. 10: 369-400.

Martinac, B., H. Zhu, A. Kubalski, X. Zhou, M. Culbertson, H. Bussey, and C. King. (1990). Yeast K1 killer toxin forms ion channels in sensitive yeast spheroplasts and in artificial liposomes. Proc. Natl. Acad. Sci. USA 87: 6228-6232.

Santos, A., M. San Mauro, C. Abrusci, and D. Marquina. (2007). Cwp2p, the plasma membrane receptor for Pichia membranifaciens killer toxin. Mol. Microbiol. 64: 831-843.

Schmitt, M. and F. Breinig. (2002). The viral killer system in yeast: from molecular biology to application. FEMS Microbiol. Rev. 26: 257.

Vondrejs, V., B. Janderová, and L. Valásek. (1996). Yeast killer toxin K1 and its exploitation in genetic manipulations. Folia Microbiol (Praha) 41: 379-393.

Examples:

TC#NameOrganismal TypeExample
1.C.6.1.1

M1-1 protoxin precursor killer toxin K1 of 316 aas and 1 TMS (Vondrejs et al. 1996; Becker and Schmitt 2017).

Yeast

YKT-K1 of Saccharomyces cerevisiae

 
1.C.6.1.2

Pre-killer Toxin of 329 aas and 1 N-terminal TMS, PMKT (PMKS-004207) (Santos et al. 2007; Belda et al. 2017). The mature toxin is a dimer of the α-subunit (63 aas) and the β-subunit (77 aas).

.  

PMKS-004207 of Pichia membranifaciens

 
1.C.6.1.3

Uncharacterized homologue of killer toxin of 310 aas and 1 N-terminal TMS.

UP of Kazachstania africana (Yeast) (Kluyveromyces africanus)

 
1.C.6.1.4

Uncharacterized killer toxin homologue of 307 aas and 1 N-rweminl TMS.

UP of Naumovozyma dairenensis (Saccharomyces dairenensis)

 
1.C.6.1.5

Uncharacterized killer toxin homologue of 303 aas and 1 N-terminal TMS.

UP of Naumovozyma castellii (Yeast) (Saccharomyces castellii)

 
1.C.6.1.6

Uncharacterized protein of 156 aas and 1 N-terminal TMS.

UP of Vanderwaltozyma polyspora

 
Examples:

TC#NameOrganismal TypeExample
1.C.6.2.1

Uncharacterized small killer toxin homologue of 173 aas and 3 or 4 TMSs in a 1 + 2 or 3 TMS arrangement.

UP of Torulaspora delbrueckii (Yeast) (Candida colliculosa)

 
1.C.6.2.2

Uncharacterized killer toxin homologue of 189 aas and 4 TMSs in a 1 + 3 TMS arrangement.

UP of Debaryomyces fabryi