Monopolar spindles can be rescued by acentrosomal spindle formation.

More can be more, based on two comprehensive RNAi studies of motors and actin-regulating proteins in fly S2 cells—a particularly robust system for RNAi techniques and a convenient cell type for cytological studies.

The first of the two papers, by Goshima and Vale (page 1003), examines the role of every fly kinesin during mitosis—the first such study done in higher eukaryotic cells. The group inhibited 25 kinesins plus dynein, individually and in combination, and examined the lines by live cell imaging. Loss of eight individual motors affected cell division, and three kinesins were absolutely required: BimC/Eg5 (which forms the bipolar spindle), Kip3 (which keeps spindle microtubules from overgrowing), and MKLP1 (which forms and maintains the central spindle necessary for cytokinesis).

The work shows that cells have a backup plan in case spindle formation fails. RNAi of several kinesins caused monopolar spindles to form, but many of these cells reverted to bipolar spindles. A small percentage of wild-type cells also formed and then rescued monopolar spindles. In both cases, acentrosomal poles were formed (as during meiosis and in dividing plant cells), probably through BimC/Eg5-mediated microtubule bundling near the chromosomes.

Double and triple mutants—easily created using RNAi—revealed that chromosome alignment on the metaphase plate is directed by three kinesins (CENP-E, Kid, and chromokinesin) that have partially overlapping functions. Chromosome alignment was completely disrupted only upon triple RNAi of all three motors.

Redundancies were also easily identified using RNAi in the second study, by Rogers et al. (page 1079), which examined the contribution of 90 actin-regulating proteins during lamella protrusion. The findings reveal that either of two independent pathways can activate SCAR, an Arp2/3-activating protein needed for lamella formation. The group also found that SCAR is degraded in the absence of the inhibitors (kette, Sra-1, and Abi), possibly to prevent uncontrolled actin polymerization. In total, seven phenotypic classes were seen from RNAi of 20 of the proteins tested in this study. Live cell imaging should reveal further insight into the function of these proteins. ▪