Novel G-protein-coupled receptor-like proteins in the plant pathogenic fungus Magnaporthe grisea

Abstract

Background: The G-protein-coupled receptors (GPCRs) are one of the largest protein families in human and other animal genomes, but no more than 10 GPCRs have been characterized in fungi. Do fungi contain only this handful or are there more receptors to be discovered? We asked this question using the recently sequenced genome of the fungal plant pathogen Magnaporthe grisea.

Results: Proteins with significant similarity to fungus-specific and other eukaryotic GPCRs were identified in M. grisea. These included homologs of known fungal GPCRs, the cAMP receptors from Dictyostelium, and a steroid receptor mPR. We also identified a novel class of receptors typified by PTH11, a cell-surface integral membrane protein required for pathogenicity. PTH11 has seven transmembrane regions and an amino-terminal extracellular cysteine-rich EGF-like domain (CFEM domain), a characteristic also seen in human GPCRs. Sixty-one PTH11-related proteins were identified in M. grisea that shared a common domain with homologs in Neurospora crassa and other fungi belonging to this subphylum of the Ascomycota (the Pezizomycotina). None was detected in other fungal groups (Basidiomycota or other Ascomycota subphyla, including yeasts) or any other eukaryote. The subclass of PTH11 containing the CFEM domain is highly represented in M. grisea.

Conclusion: In M. grisea we identified homologs of known GPCRs and a novel class of GPCR-like receptors specific to filamentous ascomycetes. A member of this new class, PTH11, is required for pathogenesis, thus suggesting roles in pathogenicity for other members. The identified classes constitute the largest number of GPCR-like proteins reported in fungi to date.

Figure 1

Gene phylogeny based on the conserved membrane-spanning PTH11-domain. The tree shown was constructed using parsimony methods. Numbers on branches represent bootstrap values based on 100 random dataset simulations. Open ovals indicate putative paralogs and filled ovals the M. grisea-N. crassa orthologs. For sequences other than the ones predicted from M. grisea and N. crassa genome sequences the GenBank accession numbers are indicated. The abbreviations for species names are indicated in parentheses after the accession numbers as follows: BG, Blumeria graminis; PA, Podospora anserina; NC, N. crassa. The product of the gene PTH11 was referred to as Pth11p in the original report. Subsequently it has been referred to as PTH11. We refer to this gene product as PTH11 in this paper and would like to propose revision of its name from Pth11p to PTH11.

Figure 2

Alignment of GPCR-like proteins. Domains conserved in (a) PTH11-, (b) cAMP-, (c) STM1- and (d) mPR-related classes are shown. Representative sequences from each class were aligned using T_Coffee [39]. The alignment was analyzed using GenDoc. We used the default setting using the conservative shading mode with similarity groups enabled. Black and the dark and light gray represent 80% or greater conserved, 60% or greater conserved, and less than 60% conserved, respectively. Conservative substitutions were counted as a single residue type. The GenBank or Swiss-Prot (SP) accession numbers or the accession numbers of the predicted proteins in the M. grisea or N. crassa genome databases are indicted on the left [21, 42]. The boundaries of each sequence used in the alignment are indicated on the right.

Figure 3

Membrane topology of M. grisea GPCR-like proteins. The figure shows representative examples from different classes with domains that are conserved with respect to other receptors of the same class. Known Pfam domains or domains conserved between the M. grisea protein and other members of the class, as shown in Figure 2, are shaded in black. The amino-acid residue numbers that mark the boundaries of these domains are given. The location of the domains on the membrane topology shown for the M. grisea protein is the same for other proteins that share these domains. For GPR1-related proteins, sequence similarity was limited to the membrane-spanning regions and MG00532.4 had sequence similarity with other animal GPCRs between the third and the fifth membrane-spanning regions (not shown in figure).