Disease relevance of KAR9

• In eukaryotic cells, the adenomatous polyposis coli/Kar9p and dynein/dynactin pathways are involved in guiding MT plus ends and MT sliding along the cortex, respectively [1].

High impact information on KAR9

• Clb4/Cdc28 kinase phosphorylates Kar9 and accumulates on the pole destined to the mother cell [2].
• Thus, Clb4/Cdc28 prevents Kar9 loading on the mother bound pole [2].
• Asymmetric loading of Kar9 onto spindle poles and microtubules ensures proper spindle alignment [2].
• The tail domain of Myo2 that binds Kar9 also interacts with secretory vesicles and vacuolar elements, making it a pivotal component of yeast cell polarization [3].
• Here, we show that Kar9 directs microtubule orientation by acting through Bim1, a conserved microtubule-binding protein [4].

Biological context of KAR9

• In the absence of KAR9, dynein-dependent spindle oscillations are evident before anaphase onset, as are postanaphase dynein-dependent pulling forces that exceed the velocity of wild-type spindle elongation threefold [5].
• In mitotic cells, GFP-Kar9p localized to a cortical dot with both mother-daughter asymmetry and cell cycle dependence [6].
• Two phosphorylation sites in Kar9p were previously identified [7].
• During mitosis in the yeast Saccharomyces cerevisiae, Kar9p directs one spindle pole body (SPB) toward the incipient daughter cell by linking the associated set of cytoplasmic microtubules (cMTs) to the polarized actin network on the bud cortex [7].
• Moreover, additive phenotypes in bud6Delta kar9Delta or bud6Delta dyn1Delta mutants underscored the separate contributions of Bud6p, Kar9p, and dynein to spindle positioning [8].

Anatomical context of KAR9

• Kar9p is a novel cortical protein required for cytoplasmic microtubule orientation in yeast [6].
• Kar9p localization was also altered by mutations in four genes, spa2Delta, pea2Delta, bud6Delta, and bni1Delta, required for normal polarization and actin cytoskeleton functions and, of these, bni1Delta affected Kar9p localization most severely [9].
• Paradoxically, +TIPs such as the EB1 orthologue Bim1 and Kar9 also associate with spindle pole bodies (SPBs), the centrosome equivalent in budding yeast [10].
• These contacts are eliminated in bud6 delta cells but are proficient in kar9 delta cells [11].

Associations of KAR9 with chemical compounds

• In the first mechanism, phosphorylation at serine 496 recruits Kar9p to one SPB [7].

Physical interactions of KAR9

• These experiments demonstrate that the interaction of cytoplasmic microtubules with the Kar9p cortical attachment site requires the microtubule-binding protein Bim1p [12].
• Here, we show that the CLIP-170 homologue Bik1p binds directly to Kar9p [13].
• We propose a model in which Kar3p couples depolymerizing microtubule plus ends to the cell cortex and the Bim1p-Kar9p protein complex maintains attachment during microtubule polymerization [14].

Enzymatic interactions of KAR9

• Kar9p is hypophosphorylated in bik1delta mutants, and Bik1p binds to both phosphorylated and unphosphorylated isoforms of Kar9p [13].

Regulatory relationships of KAR9

• The Cyclin-dependent Kinase Cdc28p Regulates Multiple Aspects of Kar9p Function in Yeast [7].
• Together, these data suggest that Bik1p promotes the phosphorylation of Kar9p on serine 496, which affects its asymmetric localization to one SPB and associated cytoplasmic MTs [13].

Other interactions of KAR9

• Genetic analysis placed BNI1 in the KAR9 pathway for nuclear migration [9].
• However, deletion of BIM1 eliminated Kar9p localization along cytoplasmic microtubules [12].
• Furthermore, the two-hybrid interaction between full-length KAR9 and the cyclin CLB5 requires BIK1 [13].
• A cooperative mechanism between Kar9p and Myo2p facilitates the pre-anaphase orientation of the spindle [15].
• CONCLUSIONS: Thus, Kar9p provides a key link between the actin cytoskeleton and microtubules early in the cell cycle [15].

Analytical, diagnostic and therapeutic context of KAR9

• By indirect immunofluorescence, cytoplasmic microtubules intersected the GFP-Kar9p dot [6].

References