Septins are a class of GTP-binding proteins conserved throughout many 
eukaryotes. Individual septin subunits associate with one another and assemble 
into heteromeric complexes that form filaments and higher-order structures in 
vivo. The mechanisms underlying the assembly and maintenance of higher-order 
structures in cells remain poorly understood. Septins in several organisms have 
been shown to be phosphorylated, although precisely how septin phosphorylation 
may be contributing to the formation of high-order septin structures is unknown. 
Four of the five septins expressed in the filamentous fungus, Ashbya gossypii, 
are phosphorylated, and we demonstrate here the diverse roles of these 
phosphorylation sites in septin ring formation and septin dynamics, as well as 
cell morphology and viability. Intriguingly, the alteration of specific sites in 
Cdc3p and Cdc11p leads to a complete loss of higher-order septin structures, 
implicating septin phosphorylation as a regulator of septin structure formation. 
Introducing phosphomimetic point mutations to specific sites in Cdc12p and Shs1p 
causes cell lethality, highlighting the importance of normal septin modification 
in overall cell function and health. In addition to discovering roles for 
phosphorylation, we also present diverse functions for conserved septin domains 
in the formation of septin higher-order structure. We previously showed the 
requirement for the Shs1p coiled-coil domain in limiting septin ring size and 
reveal here that, in contrast to Shs1p, the coiled-coil domains of Cdc11p and 
Cdc12p are required for septin ring formation. Our results as a whole reveal 
novel roles for septin phosphorylation and coiled-coil domains in regulating 
septin structure and function.