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

Abstract

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 mid1 protein (mid1p) shuttles between the nucleus and a cortical medial broad band during interphase and early mitosis. The position of this broad band, which overlies the nucleus, is linked to nuclear position even in cells with displaced or multiple nuclei. We identified and created mutations in an NLS and in two crm1-dependent NES sequences in mid1p. NES mutations caused mid1p accumulation in the nucleus and loss of function. An NLS mutations greatly reduced nuclear localization but did not perturb cytoplasmic localization or function. mid1p localization to the medial broad band was also not dependent on mid1p PH domain or microtubule and actin cytoskeletons. Overexpression 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, suggesting 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 function may be to position the plane of cell division.

Mid1/anillin and the spatial regulation of cytokinesis in fission yeast

“…Indeed, although Mid1 starts localizing at medial cortical nodes very early in interphase, even before NETO [Paoletti and Chang, 2000;Celton-Morizur et al, 2006], predefining the division site, it only starts recruiting contractile ring components at mitotic onset, triggering contractile ring assembly. As discussed earlier, Plo1 activity is necessary for the proper positioning of the division plane and for contractile ring assembly [Ohkura et al, 1995;].…”

“…3). Mid1 C-terminal PH domain is dispensable for Mid1 function [Paoletti and Chang, 2000] but has been recently shown to favor Mid1 cortical localization and may interact directly with lipids [Lee and Wu, 2012], as recently reported for anillin in animal cells [Liu et al, 2012]. Nevertheless, during interphase, cortical targeting of Mid1 is largely dependent on its interaction with the Ser/Thr kinase Cdr2 [Breeding et al, 1998;Kanoh and Russell, 1998;Almonacid et al, 2009;Moseley et al, 2009], a member of the family of SAD (synapses of amphids defective) kinases, whose founding member SAD-1 in C. elegans regulates neuronal polarity and synaptic organization.…”

“…As mitosis progresses through anaphase, Mid1 cortical nodes compact into a tight ring together with other contractile ring components. Mid1 eventually dissociates from the contractile ring as it starts to constrict [Sohrmann et al, 1996;Paoletti and Chang, 2000] (see Fig. 1).…”

“…3). The phosphorylation of Plo1 binding site also allows Plo1 localization to the contractile ring during mitosis [Almonacid et al, 2011].Contrary to other anillin-family members which exit the nucleus upon nuclear envelope breakdown, Mid1 contains two nuclear export sequences (NES) required for efficient translocation of the protein from the nucleus to the cortex upon entry into mitosis [Paoletti and Chang, 2000] (see Fig. 3).…”

“…However, the fact that the division plane positioning defects of plo1-1 mutant are not suppressed by expressing a mutant of Mid1 which cannot enter the nucleus (thus bypassing the requirements for nuclear export; see Fig. 2) also indicates that Plo1 has an additional role in activating Mid1 scaffolding function for contractile ring components [Paoletti and Chang, 2000; see below]. …”