Structure and function of a virally encoded fungal toxin from Ustilago maydis: a fungal and mammalian Ca2+ channel inhibitor.
BACKGROUND
The P4 strain of the corn smut fungus, Ustilago maydis, secretes a fungal toxin, KP4, encoded by a fungal virus (UMV4) that persistently infects its cells. UMV4, unlike most other (non-fungal) viruses, does not spread to uninfected cells by release into the extracellular milieu during its normal life cycle and is thus dependent upon host survival for replication. In symbiosis with the host fungus, UMV4 encodes KP4 to kill other competitive strains of U. maydis, thereby promoting both host and virus survival. KP4 belongs to a family of fungal toxins and determining its structure should lead to a better understanding of the function and evolutionary origins of these toxins. Elucidation of the mechanism of toxin action could lead to new anti-fungal agents against human pathogens.


RESULTS
We have determined the atomic structure of KP4 to 1.9 A resolution. KP4 belongs to the alpha/beta-sandwich family, and has a unique topology comprising a five-stranded antiparallel beta-sheet with two antiparallel alpha-helices lying at approximately 45 degrees to these strands. The structure has two left-handed beta alpha beta cross-overs and a basic protuberance extending from the beta-sheet. In vivo experiments demonstrated abrogation of toxin killing by Ca2+ and, to a lesser extent, Mg2+. These results led to experiments demonstrating that the toxin specifically inhibits voltage-gated Ca2+ channels in mammalian cells.


CONCLUSIONS
Similarities, although somewhat limited, between KP4 and scorpion toxins led us to investigate the possibility that the toxic effects of KP4 may be mediated by inhibition of cation channels. Our results suggest that certain properties of fungal Ca2+ channels are homologous to those in mammalian cells. KP4 may, therefore, be a new tool for studying mammalian Ca2+ channels and current mammalian Ca2+ channel inhibitors may be useful lead compounds for new anti-fungal agents.
Gu
F
F
Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA.
Khimani
A
A
Rane
S G
SG
Flurkey
W H
WH
Bozarth
R F
RF
Smith
T J
TJ
D000313:Adrenal Medulla / Q000187:drug effects / Q000502:physiology; D000595:Amino Acid Sequence; D000818:Animals; D000935:Antifungal Agents; D002121:Calcium Channel Blockers; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology; D018360:Crystallography, X-Ray; D006801:Humans; D008322:Mammals; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009183:Mycotoxins / Q000096:biosynthesis / Q000737:chemistry* / Q000494:pharmacology*; D016716:PC12 Cells; D018408:Patch-Clamp Techniques; D017510:Protein Folding; D017433:Protein Structure, Secondary; D051381:Rats; D012984:Software; D014407:Tumor Cells, Cultured; D014588:Ustilago / Q000187:drug effects / Q000821:virology*; D014764:Viral Proteins
D000935:Antifungal Agents; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D009183:Mycotoxins; D014764:Viral Proteins; C086592:Ustilago maydis viral toxin KP4