This entry represents a structural fold found in the killer toxins Kp4 and SMK, consisting of two left-handed split beta/alpha/beta motifs, which is rarely found in other toxins; hence, these toxins may be evolutionarily or functionally related . Killer toxins are polypeptides secreted by some fungal species that kill sensitive cells of the same or related species, often functioning by creating pores in target cell membranes.
The fungal killer toxin KP4 from the corn smut fungus, Ustilago maydis (Smut fungus), is encoded by a resident symbiotic double-stranded RNA virus, Ustilago maydis P4 virus (UmV4), within fungal cells. Unlike most killer toxins, KP4 is a single polypeptide . KP4 inhibits voltage-gated calcium channels in mammalian cells, which in turn inhibits cell growth and division by blocking calcium import. KP4 adopts a structure consisting of a two-layer alpha/beta sandwich with a left-handed crossover .
The killer toxin SMK (salt-mediated killer) from the halotolerant yeast Pichia farinosa acts to kill sensitive strains of yeast. SMK exhibits maximum activity under conditions of acidic pH and high salt concentration. It is composed of two distinct subunits, alpha and beta, which tightly interact with each other under acidic conditions, jointly folding into an ellipsoidal single domain structure belonging to the alpha/beta-sandwich family. SMK appears to function by associating with the membranes of sensitive cells .