Membrane proteins stick around

You wouldn't call it a long-term relationship, but the interaction between proteins and certain membrane lipids is not just a one-millisecond stand. As Hammond et al. discovered, the proteins remain attached to the lipids longer than researchers expected. The fi ndings explain how these lipids localize their effects. By temporarily hooking up with various proteins, inositol lipids in the plasma membrane help cells do everything from transmit signals to move. The lipids can confine their influence to a specific section of the membrane. For example, a crawling cell cranks up inositol levels at its leading edge and destroys the lipids at its tail end. But inositol-bound proteins face a dilemma. As the inositols wander around the cell membrane, the proteins need to hang on long enough to encounter other proteins and thereby transmit the inositol signal. But if the proteins cling on too long, they risk spreading their effects beyond the target area. How the proteins resolve this dilemma wasn't clear because nobody had measured how long the molecules stay attached. Using photobleaching, Hammond et al. estimated how fast protein–lipid couples moved within the membrane and how long they remained together. Riding the inositol molecules, the proteins traveled at a brisk pace of around 1 μm 2 per second, about the same speed as other researchers had measured for the lipids alone. To the team's surprise, the proteins could stick to the inositols for several seconds. As a result, a protein–inositol duo has time to search its neighborhood for potential interaction partners, but it can spread its infl uence over a few microns at the most. The researchers are now working to determine how interactions with other proteins affect the mobility of the pairs. Like DNA, centrioles need to duplicate only once per cell cycle. Rogers et al. uncover a long-sought mechanism that limits centriole copying, showing that it depends on the timely demolition of a protein that spurs the organelles' replication. Centrioles start reproducing themselves during G1 or S phase. What prevents the organelles from xeroxing themselves again and again has puzzled researchers for more than a decade. The process could be analogous to the mechanism for controlling DNA replication. There, a licensing factor preps the DNA for duplication. During DNA synthesis, the factor gets tagged with ubiquitin molecules that prompt its destruction, thus preventing another round of copying. To determine whether a similar mechanism keeps centrioles in check, …