Analysis of mid1p, a protein required for placement of the cell division site, reveals a link between the nucleus and the cell surface in fission yeast

mid1 is required for the proper placement of the contractile actin ring for cytokinesis at a medial site overlying the nucleus. Here we find that midi protein (mid1p) shuttles between the nucleus and a cortical medial broad b and during interphase and early mitosis. The position of this broad band, w hich overlies the nucleus, is linked to nuclear position even in cells with displaced or multiple nuclei. We identified and created mutations in an NL S and in two crm1-dependent NES sequences in mid1p. NES mutations caused mi d1p accumulation in the nucleus and loss of function. An NLS mutations grea tly reduced nuclear localization but did not perturb cytoplasmic localizati on or function. mid1p localization to the medial broad band was also not de pendent on mid1p PH domain or microtubule and actin cytoskeletons. Overexpr ession of mid1p produced ectopic cell growth at this band during interphase and abnormal karmellae-like nuclear membrane structures. In plo1-1, mid1p formed a medial broad band but did not incorporate into a tight ring, sugge sting that polo kinase plo1p is required for activation of mid1p function. Thus, the mid1p broad band defines a compartment at the medial cell surface , whose localization is linked to the position of the nucleus, and whose fu nction may be to position the plane of cell division.