Pattern formation in single cells (Reprinted from Trends in Biochemical Science, vol 12, Dec., 1999)

Single-cell patterning begins with an asymmetric cue that orients the axis of polarity. Despite great diversity in the types of cues, common mechanism s appear to mediate the polarizing response. Rho-family GTPases initially p rocess and reinforce polarity cues by remodelling cortical actin, and these Local asymmetries are subsequently propagated to the microtubules, membran e and secretory pathway to generate the final pattern. Homologues of the ye ast polarity genes fulfil similar functions in higher eukaryotes, revealing a fundamental conservation in how polarity arises. Unlike yeast, however, more complex eukaryotic cells can manifest multiple axes of polarity, sugge sting that additional mechanisms have evolved to generate more elaborate pa tterns.