Submitted to: Current Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2010
Publication Date: 11/30/2010
Citation: Brown, D.W. 2010. The KP4 killer protein gene family. Current Genetics. 57(1):51-62. Interpretive Summary: This paper describes the characterization of a gene family for the toxin Killer Protein 4 (KP4), which is produced by a corn smut fungus. KP4 toxins have antimicrobial activity and can inhibit a fungal plant pathogen called Fusarium. Analysis of sequence data from fungal, plant, and viral KP4 genes indicates that the gene first arose in the fungal kingdom and then moved to the plant kingdom and the virus. The function of the viral KP4 protein is to kill fungi, but despite sequence similarities, the fungal and plant KP4-like genes are unlikely to function in the same manner since they lack a critical amino acid required for activity. The expression of two KP4-like genes by the fungus Fusarium verticillioides during growth on corn suggest that these genes likely serve a function. Understanding how these two genes as well as four others in F. verticillioides function and affect the plant/fungal interaction will help agricultural and food scientists to develop novel strategies to limit or control toxin synthesis and keep our food supply safe.
Technical Abstract: Killer protein 4 (KP4) is a well studied toxin secreted by the maize smut fungus Ustilago maydis that kills sensitive Ustilago strains as well as inhibits Fusarium and plant root growth. This small, cysteine rich protein is encoded by a virus that depends on host survival for replication. KP4 functions by inhibiting calcium uptake and thereby interfering with calcium mediated signal transduction. Numerous small, cysteine rich proteins have been shown to play a critical role in fungal-plant-bacterial associations. As with KP4, some (e.g. plant defensins) are antimicrobial peptides and are an integral part of defense systems, while others (e.g. SIX proteins of Fusarium oxysporum) are critical for plant-disease response process. The discovery of six KP4-like genes in F. verticillioides precipitated efforts to understand their function and evolutionary origin. Analysis of publicly available genomic sequence identified 31 additional KP4-like genes from a range of Ascomycota, a Basidiomycota, and the moss Physcomitrella patens. Sequence comparison and phylogenetic analysis indicate that the viral KP4 and the moss and fungal KP4-like genes evolved from a common ancestor providing evidence for lateral gene transfer between kingdoms. Six genes of the 37 total genes are predicted to encode a protein with two, non-identical KP4-like domains in tandem separated by 29 to 56 amino acids. The results suggest that two independent events led to the dual-domain KP4 genes present in different lineages of the Ascomycota. Understanding the nature and function of KP4-like proteins in mycotoxin-producing species like Fusarium may help to limit plant diseases and increase food safety and food production.