Mesoscale spatial patterning in the Escherichia coli Min system: Reaction–diffusion versus mechanical communication

Abstract

Spatiotemporal patterning in biological systems is a fascinating subject. In this issue of PNAS, Ivanov and Mizuuchi (1) analyze patterning by Escherichia coli Min proteins in vitro. In vivo, coordinated pole-to-pole oscillations of MinD, a membrane-binding ATPase, and its binding partner MinE specify cell division specifically at midcell (Fig. 1 A and B ; reviewed in ref. 3). Prior studies modeled Min system patterning with a particular version of the Turing-style “reaction–diffusion” paradigm (4): reactions occur on the surface, generating solution inhomogeneities, which are counteracted by diffusion of the components in the solution (e.g., ref. 5). Ivanov and Mizuuchi (1) observe biologically relevant patterning in conditions where solution inhomogeneities cannot arise. Furthermore, because the surface-to-volume ratio of their system is orders of magnitude different from that present in vivo, bulk diffusion coefficients are unlikely to play critical roles. They conclude that patterning reactions are taking place exclusively on the surface, with the solution serving only as a homogeneous reservoir of components. Fig. 1. In vivo Min oscillations and relationship to in vitro amoebae. ( A ) Pole-to-pole oscillation of MinD-GFP. Time scale is in seconds. (Scale bar: 2 μm.) (Images courtesy of P. de Boer.) ( B ) E-ring dynamics. A peripheral ring defined by increased local density of MinE-GFP emerges at the edge of a MinD-rich polar zone (e.g., t = 0 s in A ), which it then follows during poleward recession (see text). (Images adapted from ref. 2 with permission from Macmillan Publishers Ltd: EMBO J 20:1563–1572, Copyright 2001; http://www.nature.com/emboj/index.html.) ( C and D ) Analogy between amoeba forms observed in vitro ( C Right and D ) and the in vivo E-ring/MinD polar zone state ( C Left and B ). (Scale bar: 5 μm.) (Image in C Left adapted from ref. 9 with permission from Macmillan Publishers Ltd: EMBO J 20:1563–1572, Copyright 2001; http://www.nature.com/emboj/index.html; images in … 1E-mail: kleckner{at}fas.harvard.edu.