A new report in Science describes the rapid identification and characterization of proteins that function in cytokinesis, using an approach that combines functional-proteomic and comparative-genomic analysis.

From synchronized Chinese hamster ovary (CHO) cells, Ahna Skop and colleagues isolated midbodies — microtubule-rich, transient structures that are derived from the spindle midzone and that exist after cell division, just before the daughter cells detach. The proteins in the midbody preparation were identified by a protein identification technology — MudPIT — that is used for the analysis of proteins in complex protein mixtures.

After eliminating proteins that have general housekeeping functions, 160 candidate midbody proteins remained, of which 57 were known cytokinesis proteins. Most candidate midbody proteins seemed to be conserved in evolution as 147 out of 160 proteins had clear Caenorhabditis elegans homologues. The functional relevance of the candidate midbody proteins became apparent when the authors analysed gene function by RNA interference using double-stranded (ds)RNA that corresponded to each of the C. elegans genes. Most of these dsRNAs produced a disrupted cytokinesis phenotype.

By analysing the various mutant phenotypes, the authors found that a significant percentage of midbody proteins that function in cytokinesis also produce defects in germline development when they are disrupted. They went on to show that gonad development and sterility mutant phenotypes are, in fact, frequently caused by defects in cytokinesis in the germline or in early embryos. In addition, 16 proteins that are essential for embryo and germline cytokinesis were also required for polar-body extrusion (the polarized, asymmetric cell division that occurs during meiosis). So, cytokinesis, gonad organization and polar-body extrusion seem to use a common set of proteins.

Although membrane and cytoskeletal proteins are known to be involved in cleavage-furrow formation, among other cytokinesis processes, how these proteins are recruited to the cleavage plane has been unclear. Skop and co-workers identified 40 membrane and cytoskeletal proteins, which include proteins that are involved in lipid-raft formation and vesicle trafficking. They suggest that raft-associated factors could target and activate specific membrane events in cytokinesis.

Among the midbody proteins, 24% were Golgi-associated proteins, which led the authors to speculate that there might be some parallels between cytokinesis in animals and plants, as Golgi-derived vesicles are involved in cell-wall formation after cell division in plants.

Finally, the functional analysis of midbody proteins revealed some unexpected phenotypes — 20% of the identified proteins caused defects in coordination, which implicates these proteins in muscle or neuronal development. In addition, 14% of the mammalian proteins that were identified are known to have a role in human diseases, mostly those that are associated with membrane and cytoskeletal pathologies. So this study provides plenty of starting points for further investigations — including into cytokinesis, development or disease mechanisms.