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DDB_G0291838 is highly similar to cse1 of budding yeast and CAS of metazoans. In yeast this protein has been shown to be essential for the re-export of importin alpha, see Schroeder et al, Mol Gen Genet 261 (1999). CSE stands for "chromosome segregation"; the gene was originally identified in a screen for mutations causing chromosome disjuction; the null is lethal and conditional mutants arrest with incomplete nuclear division Xiao et al, Mol Cell Biol 13, 4691 (1993).

The importin system is well known to participate in spindle assembly. The ran GEF RCC1 (regulator of chromosome condensation), normally nuclear, associates with the chromosomes after nuclear envelope breakdown and stimulates the release of cargoes necessary for spindle construction in the vicinity of the kinetochores.

Cse1 is upregulated threefold in a Dicty strain in which the retinoblastoma-like gene rblA has been disrupted. Most genes whose products function specifically in cell cycle progression are overexpressed in this strain. RanA, ranB, and several proteins with RCC1 domains are upregulated by similar factors. This suggests that importins function in mitosis in Dicty as they do in other organisms. DDB_G0291838/cse1 may well be a part of this system.

Harry MacWilliams, September 2009

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