Phosphoregulation of STIM1 Leads to Exclusion of the Endoplasmic Reticulum from the Mitotic Spindle

Cell Biology and Physiology Center, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892, USA.
Current biology: CB (Impact Factor: 9.92). 06/2012; 22(16):1487-93. DOI: 10.1016/j.cub.2012.05.057
Source: PubMed

ABSTRACT The endoplasmic reticulum (ER) undergoes significant reorganization between interphase and mitosis, but the underlying mechanisms are unknown. Stromal interaction molecule 1 (STIM1) is an ER Ca(2+) sensor that activates store-operated Ca(2+) entry (SOCE) and also functions in ER morphogenesis through its interaction with the microtubule +TIP protein end binding 1 (EB1). We previously demonstrated that phosphorylation of STIM1 during mitosis suppresses SOCE. We now show that STIM1 phosphorylation is a major regulatory mechanism that excludes ER from the mitotic spindle. In mitotic HeLa cells, the ER forms concentric sheets largely excluded from the mitotic spindle. We show that STIM1 dissociates from EB1 in mitosis and localizes to the concentric ER sheets. However, a nonphosphorylatable STIM1 mutant (STIM1(10A)) colocalized extensively with EB1 and drove ER mislocalization by pulling ER tubules into the spindle. This effect was rescued by mutating the EB1 interaction site of STIM1(10A), demonstrating that aberrant association of STIM1(10A) with EB1 is responsible for the ER mislocalization. A STIM1 phosphomimetic exhibited significantly impaired +TIP tracking in interphase but was ineffective at inhibiting SOCE, suggesting different mechanisms of regulation of these two STIM1 functions by phosphorylation. Thus, ER spindle exclusion and ER-dependent Ca(2+) signaling during mitosis require multimodal STIM1 regulation by phosphorylation.

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    • "Most interestingly, phosphorylation of STIM1 is critical to exclude the ER from the mitotic spindle during cell division [18]. Since EB1 and APC interactions are relevant to microtubule stabilization [19] [20] [21], and given the fact that STIM1 is a microtubule plus end tracking protein [9], we were interested in exploring the role of APC in SOCE. "
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    • "Accordingly, the ER associates with the mitotic spindle in dividing animal cells. More importantly, in all animal cell types examined, there is significant accumulation of the ER around the spindle poles (Bobinnec et al. 2003; Lu et al. 2009; Poteryaev et al. 2005; Puhka et al. 2007; Smyth et al. 2012; Terasaki 2000; Waterman-Storer et al. 1993). Thus, the ER is situated in close proximity to the centrosomes , and it is conceivable that in asymmetrically Fig. 4 Asymmetric organelle inheritance. "
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    ABSTRACT: The microtubule polymer grows and shrinks predominantly from one of its ends called the 'plus-end'. Plus-end regulation during interphase is well understood. However, mitotic regulation of plus-ends is only beginning to be understood in mammalian cells. During mitosis, the plus-ends are tethered to specialized microtubule capture sites. At these sites, plus-end-binding proteins are loaded and unloaded in a regulated fashion. Proper tethering of plus-ends to specialized sites is important so that the microtubule is able to translate its growth and shrinkage into pushing and pulling forces that move bulky subcellular structures. We discuss recent advances on how mitotic plus-ends are tethered to distinct subcellular sites and how plus-end-bound proteins can modulate the forces that move subcellular structures. Using end binding 1 (EB1) as a prototype plus-end-binding protein, we highlight the complex network of plus-end-binding proteins and their regulation through phosphorylation. Finally, we develop a speculative 'moving platform' model that illustrates the plus-end's role in distinguishing correct versus incorrect microtubule interactions.
    Open Biology 11/2012; 2(11):120132. DOI:10.1098/rsob.120132 · 4.56 Impact Factor
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