Editors' ChoiceCell Biology

Mad1 for Integrin Trafficking

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Science Signaling  25 Nov 2014:
Vol. 7, Issue 353, pp. ec328
DOI: 10.1126/scisignal.aaa3385

Mitotic arrest deficient 1 (Mad1) binds to unattached kinetochores during mitosis and recruits Mad2, which prevents cells from entering anaphase before chromosomes are properly attached to the spindle. During interphase, Mad1 and Mad2 colocalize in nuclei and at nuclear pores. Wan et al. found that Mad1 also localizes to perinuclear sites in HeLa cells, mouse embryonic fibroblasts, and cultured breast cancer cells during interphase. Immunofluorescence and cell fractionation showed that Mad1, but not Mad2, colocalized with markers of the Golgi apparatus and trans-Golgi network. Mad1 dispersed from its perinuclear location in HeLa cells exposed to the Golgi disruptor Brefeldin A. Although global protein trafficking through the Golgi was not affected in HeLa cells expressing a short hairpin RNA targeting Mad1 (Mad1-KD cells), the α5 integrin subunit accumulated in the Golgi of Mad1-KD cells, and there was less α5-containing integrin present at the surface of Mad1-KD cells compared with wild-type cells. Wild-type Mad1 and mutant versions of Mad1 that were deficient in binding Mad2 or unable to localize to nuclear pores coimmunoprecipitated with epitope-tagged α5 integrin. Integrins containing the α5 subunit bind to fibronectin. Whereas Mad1-KD cells adhered normally to collagen-coated plates, fewer Mad1-KD cells adhered to and spread on fibronectin-coated plates compared to wild-type cells. Mad1-KD cells exhibited impaired migration on fibronectin in cell culture wounding and transwell migration assays. Conversely, overexpressing Mad1 enhanced migration on fibronectin. Reducing the abundance of Mad2 did not cause the α5 integrin subunit to accumulate in the Golgi and had no effect on cell spreading. Furthermore, expressing the Mad2 binding–deficient or nuclear pore binding–deficient form of Mad1 rescued α5 integrin trafficking to the plasma membrane and cell migration defects in Mad1-KD cells. Thus, Mad1 is required for trafficking of α5 integrin during interphase. During mitosis, when Mad1 interacts with Mad2 and is restricted to kinetochores, Mad1 could be unavailable for promoting the trafficking of integrins through the Golgi, which may limit cell migration to interphase. Overexpression of Mad1 is associated with poor prognosis in some breast cancers, raising the possibility that these cancer cells might be particularly aggressive due to enhanced migration and metastasis.

J. Wan, F. Zhu, L. M. Zasadil, J. Yu, L. Wang, A. Johnson, E. Berthier, D. J. Beebe, A. Audhya, B. A. Weaver, A Golgi-localized pool of the mitotic checkpoint component Mad1 controls integrin secretion and cell migration. Curr. Biol. 24, 2687–2692 (2014). [Online Journal]

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