Yeast Num1p associates with the mother cell cortex during S/G2 phase and affects microtubular functions.

Abstract

Abstract. The NUM1 gene is involved in the control of nuclear migration in Saccharornyces cerevisiae. The content of NUM1 mRNA fluctuates during the cell cycle, reaching a maximum at S/G2 phase, and the translation product Numlp associates with the cortex of mother cells mainly during S, G2, and mitosis, as seen by indirect immunofluorescence. The nuclear spindle in NUMl-deficient large-budded cells often fails to align along the mother/bud axis, while abnormally elongated astral microtubules emanate from both spindle pole bodies. A numl null mutation confers temperature sensitivity to the cold-sensitive a-tubulin mutant tubl-1, and shows synthetic lethality with the 13-tubulin mutant alleles tub2-402, tub2-403, tub2-404, and tub2-405. Deletion mapping has defined three functionally important Numlp regions: a potential EF hand Ca 2+ binding site, a cluster of potential phosphorylation sites and a pleckstrin homology domain. The latter domain appears to be involved in targeting Numlp to the mother cell cortex. Our data suggest that the periodically expressed NUM1 gene product controls nuclear migration by affecting astral microtubule functions.

Endoplasmic reticulum-associated mitochondria–cortex tether functions in the distribution and inheritance of mitochondria

“…Consistent with previously published observations, multiple Num1 molecules were localized to clusters positioned at the cortex ( Fig. 2A) (13,16). In every case, Num1 clusters were observed adjacent to regions of the mitochondrial network that were localized persistently at the cell cortex ( Fig.…”

“…Together the localization of Num1 clusters, their association with mitochondria, and the behavior of mitochondria in their proximity suggest that Num1 functions to tether mitochondria to the cortex physically to ensure accurate distribution of the organelle between mother and daughter cells. Significantly, Num1 clusters are observed in the mother cell and larger buds but are absent in small buds (13,16). Thus, early in the cell cycle the mitochondrial tethering activity of Num1 is predominantly mother-cell specific.…”

The role of mitochondria in anchoring dynein to the cell cortex extends beyond clustering the anchor protein

“…The CC domain interacts directly with the mitochondrial membrane and is also required for the Num1-dynein interaction [10–12]. The PH domain, which binds with high specificity to PI 4,5 P 2 , targets Num1 to the plasma membrane [13,14], where the protein is found in clusters as well as in a pool that is diffusely localized along the plasma membrane and with cortical ER [3,5,10,15,16]. Cluster formation, which is dependent on the CC domain and an interaction with mitochondria (Figure 1(a)), is critical for the function of Num1 in both mitochondria and dynein anchoring [10,11,17].…”

Interdependence of Filamentous Actin and Microtubules for Asymmetric Cell Division

“…Cells deficient in Num1p often fail to align the spindle along the mother-bud axis and undergo nuclear division in the mother cell. As for mutants in the dynactin complex, the astral microtubules are abundant and extend into the bud (Farkasovsky and Küntzel, 1995). It is striking that Num1p localizes to the mother cell cortex during S, G2, and M phase (Figure 2).…”

“…Mutations in both the dynactin complex (Li et al, 1993;McMillan and Tatchell, 1994;Muhua et al, 1994;Clark and Meyer, 1994) and in two proteins, Num1p (Farkasovsky and Küntzel, 1995) and Karp9 (Miller and Rose, 1998), were found to result in defects in spindle positioning and orientation.…”