Isolation and analysis of functional homologues of the secretion-related SAR1 gene of Saccharomyces cerevisiae from Aspergillus niger and Trichoderma reesei

Cell Biology of Hyphal Growth

Filamentous fungi are a large and ancient clade of microorganisms that occupy a broad range of ecological niches. The success of filamentous fungi is largely due to their elongate hypha, a chain of cells, separated from each other by septa. Hyphae grow by polarized exocytosis at the apex, which allows the fungus to overcome long distances and invade many substrates, including soils and host tissues. Hyphal tip growth is initiated by establishment of a growth site and the subsequent maintenance of the growth axis, with transport of growth supplies, including membranes and proteins, delivered by motors along the cytoskeleton to the hyphal apex. Among the enzymes delivered are cell wall synthases that are exocytosed for local synthesis of the extracellular cell wall. Exocytosis is opposed by endocytic uptake of soluble and membrane-bound material into the cell. The first intracellular compartment in the endocytic pathway is the early endosomes, which emerge to perform essential additional functions as spatial organizers of the hyphal cell. Individual compartments within septated hyphae can communicate with each other via septal pores, which allow passage of cytoplasm or organelles to help differentiation within the mycelium. This article introduces the reader to more detailed aspects of hyphal growth in fungi.

Characterisation of CwpA, a putative glycosylphosphatidylinositol-anchored cell wall mannoprotein in the filamentous fungus Aspergillus niger

Glycosylphosphatidylinositol (GPI)-anchored proteins in fungi are found at the cell surface, either as plasma membrane proteins (GPI-PMPs) or attached by a remnant of the GPI-anchor to the cell wall (GPI-CWPs). GPI-CWPs can be extracted from the cell wall by treatment with hydrofluoric acid (HF), which cleaves the phosphodiester bond that is present in the remnant of the GPI-anchor. The filamentous fungus Aspergillus niger contains at least seven HF-extractable cell wall mannoproteins. One gene encoding an HF-extractable cell wall mannoprotein, cwpA, was cloned and further characterised. The protein sequence of CwpA indicated the presence of two hydrophobic signal sequences both at the N-terminus and C-terminus of the protein, for entering the ER and the addition of a GPI-anchor, respectively. A CwpA-specific antiserum was raised and in combination with fractionation experiments, we show that this protein was abundantly present as an HF-extractable protein in the cell wall of A. niger.

Identification of growth phenotype-related genes in Aspergillus oryzae by heterologous macroarray and suppression subtractive hybridization

Aspergillus oryzae requires polarized growth for colonization of solid substrates, and this growth phenotype differs from that seen in liquid medium. Various experimental approaches were used to identify genes that are differentially expressed when A. oryzae is grown on wheat kernels and in a wheat-based liquid medium. Hybridization of A. oryzae RNAs to a macroarray bearing cDNAs isolated from a library representing at least 16% of the total number of A. niger genes identified 14 differentially expressed cDNA clones, showing that heterologous macroarray analysis with an A. niger cDNA library can be used to identify regulated gene transcripts in the related species A. oryzae. Moreover, Northern analysis with a selection of eight probes for A. niger genes encoding proteins involved in morphological development and cell wall biosynthesis identified five more differentially expressed genes. A suppression subtractive hybridization procedure revealed another 12 differentially expressed genes. The results presented show that, of the 29 identified genes which are expressed at higher levels during growth on wheat kernels, six encode proteins that are functionally related to polarized growth, four encode products known to be involved in morphogenesis, three code for proteins related to cell wall composition, and nine of the cDNA clones encode novel proteins. These findings pinpoint genes associated with the changes in cellular morphogenesis seen in A. oryzae grown on wheat kernels as opposed to wheat-based liquid medium.

Characterization of secretory genes ypt1/yptA and nsf1/nsfA from two filamentous fungi: induction of secretory pathway genes of Trichoderma reesei under secretion stress conditions

Two genes involved in protein secretion, encoding the Rab protein YPT1/YPTA and the general fusion factor NSFI/NSFA, were characterized from two filamentous fungi, Trichoderma reesei and Aspergillus niger var. awamori. The isolated genes showed a high level of conservation with their Saccharomyces cerevisiae and mammalian counterparts, and T. reesei ypt1 was shown to complement yeast Ypt1p depletion. The transcriptional regulation of the T. reesei ypt1, nsf1, and sar1 genes, involved in protein trafficking, was studied with mycelia treated with the folding inhibitor dithiothreitol (DTT) and with brefeldin A, which inhibits membrane traffic between the endoplasmic reticulum and Golgi complex. The well-known inducer of the yeast and T. reesei unfolded protein response (UPR), DTT, induced the nsf1 gene and the protein disulfide isomerase gene, pdi1, in both of the experiments, and sar1 mRNA increased in only one experiment under strong UPR induction. The ypt1 mRNA did not show a clear increase during DTT treatment. Brefeldin A strongly induced pdi1 and all of the intracellular trafficking genes studied. These results suggest the possibility that the whole secretory pathway of T. reesei could be induced at the transcriptional level by stress responses caused by protein accumulation in the secretory pathway.

Traffic-independent function of the Sar1p/COPII machinery in proteasomal sorting of the cystic fibrosis transmembrane conductance regulator

Newly synthesized proteins that do not fold correctly in the ER are targeted for ER-associated protein degradation (ERAD) through distinct sorting mechanisms; soluble ERAD substrates require ER-Golgi transport and retrieval for degradation, whereas transmembrane ERAD substrates are retained in the ER. Retained transmembrane proteins are often sequestered into specialized ER subdomains, but the relevance of such sequestration to proteasomal degradation has not been explored. We used the yeast Saccharomyces cerevisiae and a model ERAD substrate, the cystic fibrosis transmembrane conductance regulator (CFTR), to explore whether CFTR is sequestered before degradation, to identify the molecular machinery regulating sequestration, and to analyze the relationship between sequestration and degradation. We report that CFTR is sequestered into ER subdomains containing the chaperone Kar2p, and that sequestration and CFTR degradation are disrupted in sec12ts strain (mutant in guanine-nucleotide exchange factor for Sar1p), sec13ts strain (mutant in the Sec13p component of COPII), and sec23ts strain (mutant in the Sec23p component of COPII) grown at restrictive temperature. The function of the Sar1p/COPII machinery in CFTR sequestration and degradation is independent of its role in ER-Golgi traffic. We propose that Sar1p/COPII-mediated sorting of CFTR into ER subdomains is essential for its entry into the proteasomal degradation pathway. These findings reveal a new aspect of the degradative mechanism, and suggest functional crosstalk between the secretory and the degradative pathways.

Cloning and characterization of the bipA gene encoding ER chaperone BiP from Aspergillus oryzae

We describe the cloning and characterization of the ER localized chaperone (BiP) encoding gene from the filamentous fungus Aspergillus oryzae. The BiP encoding gene, designated bipA, has three introns and encodes a protein with 672 amino acids, which has a high homology with various BiP-like proteins. Sequences resembling heat shock elements (HSEs) and unfolded protein response (UPR) elements, as found in the Saccharomyces cerevisiae KAR2 promoter, are present in the 5' no coding region of the bipA gene. Transcription of the bipA gene was increased by heat shock or tunicamycin treatment. Expression of bipA cDNA partially complemented the temperature-sensitive growth of kar2 mutant alleles of S. cerevisiae. These results indicate that the bipA gene product plays a role as BiP in A. oryzae.