Pushing the envelope in mitosis

Abstract

One of the first events of mitosis is nuclear envelope (NE) breakdown. During prophase, the NE dissociates into vesicles that disperse throughout the cell. Recent work, however, suggests that a NE-associated protein plays an important role in chromosome segregation in Drosophila, in spite of the fact that the NE is not present during mitosis.The Su(var)2-10 gene of Drosophila was first identified because mutants in this gene are defective in position effect variegation (PEV), a process in which genes are silenced by nearby heterochromatin. Karpen and coworkers1xThe Drosophila Su(var)2-10 locus regulates chromosome structure and function and encodes a member of the PIAS protein family. Hari, K.L. et al. Genes Dev. 2001; 15: 1334–1348Crossref | PubMed | Scopus (120)See all References1 found that Su(var)2-10 mutations also showed defects in minichromosome transmission, suggesting that this gene plays a role in chromosome segregation. Cytological analysis of mutant animals lacking wild-type Su(var)2-10 revealed that mitosis is highly aberrant. In metaphase, chromosomes are large and puffy, indicating a defect in chromosome condensation. In anaphase, the Su(var)2-10 mutation caused chromosome bridges and fragmentation. Given that Su(var)2-10 mutations cause defects in a heterochromatin-based process (PEV) as well as chromosome condensation and segregation, the obvious guess would be that it encodes a chromosomal protein. Indeed, cloning and sequencing revealed that Su(var)2-10 contained potential DNA-binding motifs.Surprisingly, immunofluorescence showed that Su(var)2-10 does not localize to mitotic chromosomes. Instead, during interphase, the protein is concentrated mainly at the nuclear envelope, colocalizing with nuclear lamins. Although there are also some intranuclear foci, these do not generally colocalize with chromatin. If we take these data at face value and conclude that the primary site of Su(var)2-10 function is the nuclear envelope, we are left with an interesting puzzle: how does a protein localized on the nuclear envelope influence so many different aspects of chromosome behavior?A possible solution to this dilemma came from examining the interphase nuclear architecture in Su(var)2-10 mutants. Fluorescence in situ hybridization (FISH) using telomeric probes, revealed that the telomere clustering seen in normal cells, as well as the normal association of telomeres with the nuclear envelope, was disrupted in the mutants. Thus, Su(var)2-10 might play a role in mediating interactions between specific chromosomal regions, such as telomeres, with the nuclear envelope. Defects in Su(var)2-10 could therefore have far-reaching consequences for phenomena such as PEV that involve long-range interactions within the nucleus and are thus sensitive to changes in nuclear organization. But what about mitosis? Mitotic chromosome condensation in Drosophila initiates at a small number of condensation foci that are always associated with the nuclear envelope. If Su(var)2-10 plays a role in this interaction, mutations in the protein might interfere with the early stages of prophase chromosome condensation, leading to defects that only become apparent later in mitosis.