Tandem KH domains of Khd4 recognize AUACCC and are essential for regulation of morphology as well as pathogenicity inUstilago maydis

Abstract

RNA-binding proteins constitute key factors of the post-transcriptional machinery. These regulatory proteins recognize specific elements within target transcripts to promote, for example, maturation, translation, or stability of mRNAs. In Ustilago maydis, evidence is accumulating that post-transcriptional processes are important to determine pathogenicity. Deletion of khd4, encoding a predicted RNA-binding protein with five K homology (KH) domains, causes aberrant cell morphology and reduced virulence. Here, we demonstrate that Khd4 recognizes the sequence AUACCC in vivo via its tandem KH domains 3 and 4. This sequence most likely functions as a regulatory RNA element in U. maydis, since it accumulates in 39 untranslated regions. Consistently, an independent mRNA expression profiling approach revealed that the binding motif is significantly enriched in transcripts showing altered expression levels in khd4D strains. Since the vast majority of potential Khd4 target mRNAs exhibit increased amounts in deletion mutants, Khd4 might promote mRNA instability. Mutants that fail to bind AUACCC resemble deletion mutants, which exhibit altered cell morphology, disturbed filamentous growth, and severely reduced virulence. Hence, RNA binding is essential for function of Khd4, stressing the importance of post-transcriptional control in regulating morphology and pathogenicity.

A glycine-rich protein MoGrp1 functions as a novel splicing factor to regulate fungal virulence and growth in Magnaporthe oryzae

“…Interestingly, homologs of Rbp35 are found in many filamentous fungi, but are absent in yeast, plants, or animals, suggesting that Rbp35 is restricted to filamentous fungi. Accordingly, we speculate that filamentous fungi may contain many other similar RNA-binding proteins that control fungal development and pathogenicity (Vollmeister et al 2009). In U. maydis, among 18 RNA-binding proteins, Khd4 and Rrm4 were found to be required for fungal growth and virulence, and Khd1 contributed to cold-sensitive growth (Becht et al 2005).…”

Fungal development of the plant pathogenUstilago maydis

“…It constitutes the founding member of a multi‐KH domain family of fungal proteins with representatives in other pathogens such as C. neoformans and C. albicans . Khd4 recognizes the sequence AUACCC via the two central tandem‐KH domains (KH3 and KH4; Vollmeister et al , 2009). Interestingly, this motif is enriched in the 3′ untranslated region of mRNAs and a number of transcripts, which exhibit altered expression in khd4 Δ strains, contain this binding site.…”

RNA Biology in Fungal Phytopathogens

“…Khd4 is a multi-KH-domain protein with homologs in other pathogens such as Cryptococcus neoformans and Candida albicans . It specifically binds the sequence AUACCC and is most likely involved in regulation of mRNA stability [10]. This is supported by the observation that Khd4 localizes to processing bodies (unpublished observation), which are known centers for mRNA degradation (Figure 1, #3).…”