Polar Explorers Membrane Proteins that Determine Division Site Placement

tern of MinC and MinD is dramatically changed when The ability to place the division site at a specified posiminE is also expressed. MinC and MinD move to one tion within the cell is a fundamental property of most end of the cell to form a membrane-associated polar living organisms. One important factor in division site zone that extends from the end of the cell to a position placement in both eukaryotic and prokaryotic cells is near midcell (Figure 1g). At approximately the same time, the positioning of daughter chromosomes. This plays the membrane-associated MinE assembles into a ringan important role in determining the normal placement like structure that is localized at or close to the medial of the division septum between the two progeny chroedge of the MinCD polar zone, while the remainder of mosomal complements, which are usually positioned at the membrane-associated MinE assembles into a polar either side of midcell. In addition, studies in bacteria zone at the same end of the cell as the MinCD polar have revealed a second mechanism to ensure that divizone (Figure 1a). It is not known whether assembly of sion only occurs at the midpoint of the cell. This second the E-ring and polar zone are coupled or whether assemmechanism is exemplified by the min system of E. coli. bly of one structure precedes the other. The min system acts as a negative regulatory element The presence of MinC at a single cell pole as dethat restricts septation to the midcell site by preventing scribed above does not explain the ability of the MinC formation of division septa at other sites within the cell. division inhibitor to block aberrant septation events at Important advances in recent years have provided a both poles. This apparent paradox is explained by the growing understanding of the site-specificity function remarkable ability of the membrane-associated MinC of the min system, revealing a far more dynamic process (together with MinD) to oscillate rapidly from one pole than originally appreciated. The paper by Hu and Lutto the other (Figures 1g–1l) (Hu and Lutkenhaus, 1999; kenhaus (2001) in the June issue of Molecular Cell adds Raskin and de Boer, 1999a, 1999b). The rate of oscillaimportant new insight into the molecular mechanism tion is influenced by the cellular MinE/MinD ratio and used by the min system to ensure proper placement of by growth conditions, but generally occurs with a perithe division site. odicity of less than 90 s so that the MinCD polar zone The MinCDE System undergoes many oscillations during each division cycle. The three Min proteins—MinC, MinD, and MinE—carry As a result, each pole is free of MinCD for a relatively out different functions (for additional references see short period of time before the next wave of MinCD Rothfield et al., 1999). MinC is an inhibitor of septation returns to reassemble the polar zone. This unexpected that acts by preventing the tubulin-like FtsZ protein from phenomenon, discovered by Raskin and de Boer forming a stable cytokinetic ring, the first known step (1999b), is likely to explain the ability of the MinC division in formation of the prokaryotic division machinery (Hu inhibitor to block division at both poles, based on the et al., 1999; Justice et al., 2000). However, the MinC assumption that the length of time that the polar sites division inhibitor lacks site specificity when expressed are free of the division inhibitor during each oscillatory in the absence of MinD and MinE. As a result, when cycle is too short to permit formation of a stable FtsZ expressed at high levels in the absence of MinD and ring. MinE plays a central role in these events, being MinE, MinC not only blocks septation at aberrant sites required for formation of the oscillating MinCD polar near the cell poles but also prevents septation at the zones. normal midcell site. Both MinD and MinE are needed to The original observation that the E-ring was located give site-specificity to the MinC division inhibitor. near midcell suggested that the only role of MinE might MinD is a peripheral membrane protein that acts as be to prevent the division inhibitor from acting at midcell. a membrane assembly protein for both MinC and MinE. However, recent studies have shown that the E-ring is MinD can associate with the membrane in the absence not fixed at midcell, but is a mobile structure (Fu et al.,