Normal septin ring assembly (left) is disturbed in a Cdc42p mutant that hydrolyzes GTP slowly (right).

Cellular GTPases are generally placed into one of two categories: signaling switches such as the oncogene ras or assembly factors such as the translation elongation factor EF-Tu (EF-1a in eukaryotes). But on page 315, Gladfelter et al. show that yeast Cdc42p, a well-characterized ras-like switch, can also behave as an EF-Tu–like assembly factor. The work is the first description of a GTPase changing hats in this way, and it suggests that GTPases may defy strict categorization.Like all ras-like GTPases, Cdc42p activates downstream effectors when bound to GTP, and stops signaling after it hydrolyzes the GTP to GDP. In an effort to study Cdc42p activity during the assembly of the septin ring, a structure important in yeast budding, the authors characterized two cdc42 mutants with defects in septin ring assembly. Surprisingly, both of the recessive mutants exhibited reduced GTP hydrolysis, and overexpression of a Cdc42p GTPase activating protein could overcome their septin ring defects. If Cdc42p acted only as a ras-like GTPase, mutations that reduce GTP hydrolysis should lead to constitutive signaling and be dominant. The authors propose that, in addition to its ras-like signaling capabilities, Cdc42p can function as an EF-Tu–like factor that uses GTP hydrolysis to drive the assembly of a structure. It remains to be seen whether other signaling GTPases are similarly multitalented. ▪