Aspergillus nidulans hypB encodes a Sec7-domain protein important for hyphal morphogenesis

Area of Science:

• Molecular Biology
• Mycology
• Cell Biology

Background:

• The hypB5 temperature-sensitive allele in Aspergillus nidulans causes a phenotype of wide, highly-branched hyphae.
• This phenotype suggests a role for the hypB gene in regulating hyphal morphology and cell growth.

Purpose of the Study:

• To clone and characterize the hypB gene responsible for the observed hyphal phenotype.
• To elucidate the function of the hypB gene product in fungal cell biology.

Main Methods:

• Phenotypic complementation using a genomic plasmid library to clone the hypB locus.
• Sequence analysis of the hypB5 allele and the predicted gene AN6709.
• Gene deletion of AN6709 to assess essentiality and phenotypic effects.
• Microscopic observation of hyphal morphology under different temperature conditions.

Main Results:

• The hypB locus was identified as gene AN6709, encoding a putative Sec7 domain protein involved in COPI-mediated vesicle transport.
• The hypB5 allele contains a mutation leading to an alanine-to-proline change near the Sec7p nucleotide binding site, affecting protein stability.
• Deletion of AN6709 (hypB) was not essential, but AN6709Delta mutants exhibited a similar hyphal phenotype to hypB5.
• Shifting hypB5 mutants to a restrictive temperature induced tip cell death and basal cell growth activation.

Conclusions:

• The hypB gene (AN6709) encodes a Sec7 domain protein critical for normal hyphal growth and cell integrity in Aspergillus nidulans.
• The hypB5 mutation impairs retrograde Golgi to ER transport, leading to characteristic morphological defects.
• Temperature-induced stress on hypB5 mutants reveals distinct responses in tip and basal cells, highlighting the gene's role in polarized growth and cell cycle regulation.