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 t he tip of an emerging bud is temporally limited, filamentous fungi exhibit constitutive polarized growth of the hyphal tip. In many fungi, including A shbya gossypii, polarized growth is reinforced by a process called hyphal m aturation. Hyphal maturation refers to the developmental switch from slow-g rowing hyphae of young mycelium to fast-growing hyphae of mature mycelium. This process is essential for efficient expansion of mycelium, We report fo r the first time on the identification and characterization of a fungal gen e important for hyphal maturation. This novel A. gossypii gene encodes a pr esumptive PAK (p21-activated kinase)-like kinase, Its closest homolog is th e S, cerevisiae Cla4 protein kinase; the A. gossypii protein is therefore c alled AgCla4p, Agcla4 deletion strains are no longer able to perform the de velopmental switch from young to mature hyphae, and GFP (green fluorescent protein)-tagged AgCla4p localizes with much higher frequency in mature hyph al tips than in young hyphal tips. Both results support the importance of A gCla4p in hyphal maturation. AgCla4p is also required for septation, indica ted by the inability of Agcla4 deletion strains to properly form actin ring s 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 deve loped network of actin cables can be seen in Agcla4 deletion strains. The p ossibility that AgCla4p may be involved in regulatory mechanisms that contr ol the dynamics of the actin patches and/or actin cables is discussed.