The proliferation disrupter (prod) gene of Drosophila melanogaster encodes a novel protein associated with centromeric chromosomal regions that is required for chromatin condensation and cell viability. We have examined the binding of the Prod protein to DNA in vitro. Co-immunoprecipitation experiments demonstrate that Prod is a DNA-binding protein that specifically recognizes the 10 bp AGAATAACAT satellite repeat of D.melanogaster. Footprinting experiments show that the protein interacts with a 5–8 bp target sequence in each 10 bp repeat and suggest that it can mediate condensation of this satellite into a superhelix. Gel retardation experiments indicate that Prod does not have a well defined DNA-binding domain and it binds the satellite in a co-operative manner, probably forming Prod multimers. Since Prod localizes to both heterochromatin and euchromatin in vivo, we discuss the possibility that the ability of pre-existing euchromatic proteins to bind DNA in a co-operative manner, might be a prerequisite of satellite compaction and satellite amplification, thereby providing a basic factor in heterochromatin evolution.

Nuclear migration is a critical component of many cellular and developmental processes. The nuclear envelope forms a barrier between the cytoplasm, where mechanical forces are generated, and the nucleoskeleton. The LINC complex consists of KASH proteins in the outer nuclear membrane and SUN proteins in the inner nuclear membrane that bridge the nuclear envelope. How forces are transferred from the LINC complex to the nucleoskeleton is poorly understood. The Caenorhabditis elegans lamin, LMN-1, is required for nuclear migration and interacts with the nucleoplasmic domain of the SUN protein UNC-84. This interaction is weakened by the unc-84(P91S) missense mutation. These mutant nuclei have an intermediate nuclear migration defect-live imaging of nuclei or LMN-1::GFP shows that many nuclei migrate normally, others initiate migration before subsequently failing, and others fail to begin migration. At least one other component of the nucleoskeleton, the NET5/Samp1/Ima1 homologue SAMP-1, plays a role in nuclear migration. We propose a nut-and-bolt model to explain how forces are dissipated across the nuclear envelope during nuclear migration. In this model, SUN/KASH bridges serve as bolts through the nuclear envelope, and nucleoskeleton components LMN-1 and SAMP-1 act as both nuts and washers on the inside of the nucleus.