A PAK-like protein kinase is required for maturation of young hyphae and septation in the filamentous ascomycete Ashbya gossypii

Filamentous fungi grow by hyphal extension, which is an extreme example of polarized growth. In contrast to yeast species, where polarized growth of the tip of an emerging bud is temporally limited, filamentous fungi exhibit constitutive polarized growth of the hyphal tip. In many fungi, including Ashbya gossypii, polarized growth is reinforced by a process called hyphal maturation. Hyphal maturation refers to the developmental switch from slow-growing hyphae of young mycelium to fast-growing hyphae of mature mycelium. This process is essential for efficient expansion of mycelium. We report for the first time on the identification and characterization of a fungal gene important for hyphal maturation. This novel A. gossypii gene encodes a presumptive PAK (p21-activated kinase)-like kinase. Its closest homolog is the S. cerevisiae Cla4 protein kinase; the A. gossypii protein is therefore called AgCla4p. Agcla4 deletion strains are no longer able to perform the developmental switch from young to mature hyphae, and GFP (green fluorescent protein)-tagged AgCla4p localizes with much higher frequency in mature hyphal tips than in young hyphal tips. Both results support the importance of AgCla4p in hyphal maturation. AgCla4p is also required for septation, indicated by the inability of Agcla4 deletion strains to properly form actin rings and chitin rings. Despite the requirement of AgCla4p for the development of fast-growing hyphae, AgCla4p is not necessary for actin polarization per se, because tips enriched in cortical patches and hyphae with a fully developed network of actin cables can be seen in Agcla4 deletion strains. The possibility that AgCla4p may be involved in regulatory mechanisms that control the dynamics of the actin patches and/or actin cables is discussed.