Repair orders

Viruses catch a wave cientists know from looking at fixed cells that viruses frequently associate with filopodia and microvilli. Using video microscopy, Lehmann et al. (page 317) find that after a virus attaches to the filopodial membrane, it surfs down the plasma membrane toward the cell body. Entry into the cell occurs only after the virus has reached the base of the filopodium. Retroviruses labeled with YFP initially contacted the filopodia of cells grown in culture. After a brief period of moving randomly on the filipodial surface, the viruses moved steadily in a retrograde fashion. Once the virus reached the cell body, the viral and host cell membranes fused, as detected by the diffusion of the fluorescently labeled envelope protein of the virus. Vesicular stomatitis viruses, pHdependent viruses that require endocytosis before membrane fusion, also surfed to the base of the filopodium, where they colocalized with clathrin. S Repair orders large and varied group of proteins accumulates at the site of double strand DNA breaks (DSBs). But just what role each plays in the assembly process and in what order they arrive has been unclear. Bekker-Jensen et al. clear up one controversy on page 201, demonstrating that Mdc1/NFBD1 precedes 53BP1’s arrival and is required for 53BP1’s stable association with the chromatin surrounding DSBs. Mdc1 and 53BP1 are two of the earliest proteins to accumulate at DSBs, but recent studies reported conflicting data as to how the proteins influence each other’s binding at damaged chromosomes. To find out, Bekker-Jensen et al. followed the assembly process in real time. They used a micro-laser to generate DSBs in tissue culture cells and started time-lapse imaging immediately. A