How does the cell determine where its middle is? In fission yeast, the position of the nucleus defines the "cell middle." We are studying how the nucleus is positioned in the middle of the cell by dynamic interphase microtubules. Interphase microtubules are organized in 3-4 anti-parallel microtubule bundles arranged along the long axis of the cell, with microtubule plus ends facing both the cell tips and minus ends near the middle of the cell. The microtubule bundles are organized from peri-nuclear medial microtubule-organizing centers (iMTOCs) that may function as nuclear attachment sites. Microtubules are highly dynamic. When microtubules grow to the cell tips, they continue to grow for about 1.6 min and then shrink back the medial region of the cell before growing out again, with each cycle taking 4-6 min. Microtubule catastrophe occurs only at the cell tips.

We have proposed that the nucleus is positioned in the middle by pushing forces exerted by growing microtubules. The nuclear envelope is constantly moved or deformed by these dynamic microtubules. Growing microtubules push the nucleus each time the microtubule hits the cell tip. A balance of these simple pushing microtubule forces provides a mechanism to center the nucleus.

Fission yeast cells possess three types of microtubule organizing centers (MTOCs in the mitotic cell cycle: 1) the spindle pole body, which nucleates microtubules for the mitotic spindle and one of the interphase microtubule bundles, 2) the iMTOC, which organizes most of the interphase microtubule bundles from the outer nuclear envelope, and 3) the eMTOC, which organizes microtubules at cell division from the cell division site. We are particularily interested in defining the composition and structure of the iMTOC: how are these microtubules nucleated, bundled and attached to the nuclear envelope?

One project focuses on a novel factor rsp1p, a J-domain protein involved in disassembly of the eMTOC. In rsp1 mutant cells, the eMTOC persists in interphase cells and forms an abnormal microtubule aster that interferes with spatial regulation in the cell. rsp1p associates with the chaperone hsp70 and another eMTOC protein, suggesting that rsp1 may bring about MTOC disassembly through recruitment of a chaperone system.