Linking Telomeres to the Spindle Pole Body

OK I haven't been writing anything on papers I've read recently, to make up for this here is a brief summary of a neat paper on Sad1:

1- Sad1 is a homologue of the SUN proteins in S. pombe (fission yeast). In higher eukaryotes the SUNs are inner-nuclear membrane proteins that link to KASH domain containing proteins that reside in the outer nuclear envelope. These KASH containing proteins, such as Nesprins, then link to cytoskeletal elements in the cytoplasmic space and are involved in moving the nucleus to various intracellular spaces.

So what is Sad1 doing in pombe? During Meiosis the ends of chromosomes, or telomeres, are tethered to the spindle pole body (SPB; the centrosome equivalent in yeast). This arrangement of chromosomes is called a bouquet. How is this done? The telomeric protein, Rap1p, links to two molecules named Bqt1p and Bqt2p that are only expressed in meiosis and can bind to Sad1p. The bridging complex (Rap1p, Bqt1p, Bqt2p, Sad1p) forms on the inner nuclear membrane, away from the spindle pole body. The authors speculate that to pull the telomeres to the SPB, a Sad1p binding protein that is present on the outer nuclear envelope (kms1p) links to the microtubule motor dynein that could drag the complex to the microtubule minus ends found at the SPB, but they have no proof of this. In fact I've been told that dynein is not required to form bouquets in pombe, however it may play other roles in meiosis.

Now why is this cool? Potentially we have the first molecular link between events occurring inside and outside the nucleus. Moreover, this link traverses two nuclear membranes allowing polymers and motor proteins from the cytosol to drag around nuclear sturctures, such as chromosomes.

Ref:Yuji Chikashige, Chihiro Tsutsumi, Miho Yamane, Kasumi Okamasa, Tokuko Haraguchi, and Yasushi Hiraoka. Meiotic Proteins Bqt1 and Bqt2 Tether Telomeres to Form the Bouquet Arrangement of Chromosomes. Cell (06) 125: 59-69