The BRCA1/BARD1 Heterodimer Modulates Ran-Dependent Mitotic Spindle Assembly

Abstract

The heterodimeric tumor-suppressor complex BRCA1/BARD1 exhibits E3 ubiquitin ligase activity and participates in cell proliferation and chromosome stability control by incompletely defined mechanisms. Here we show that, in both mammalian cells and Xenopus egg extracts, BRCA1/BARD1 is required for mitotic spindle-pole assembly and for accumulation of TPX2, a major spindle organizer and Ran target, on spindle poles. This function is centrosome independent, operates downstream of Ran GTPase, and depends upon BRCA1/BARD1 E3 ubiquitin ligase activity. Xenopus BRCA1/BARD1 forms endogenous complexes with three spindle-pole proteins, TPX2, NuMA, and XRHAMM--a known TPX2 partner--and specifically attenuates XRHAMM function. These observations reveal a previously unrecognized function of BRCA1/BARD1 in mitotic spindle assembly that likely contributes to its role in chromosome stability control and tumor suppression.

Interplay of mevalonate and Hippo pathways regulates RHAMM transcription via YAP to modulate breast cancer cell motility

“…RHAMM functions as cell-surface HA receptor and cytoplasmic mitotic spindle binding protein (4,5). It mediates tumor progression through CD44 partnership and promotes genomic instability via regulating the mitotic spindle/centrosome integrity (18,19). This RHAMMregulated activation process results in increased cell-surface expression of CD44 and enhanced activation of ERK1/2.…”

“…Overexpression of RHAMM causes transformation and promotes breast cancer cell migration and invasion (BCCMI), and its expression is up-regulated in a variety of human tumors, including breast and endometrial carcinomas (6)(7)(8), gastrointestinal cancers (9, 10), prostate cancer (11), aggressive fibromatosis (i.e., desmoid tumor) (12), lung and liver cancer (13,14), glioma (15), and B-cell malignancies (16,17). RHAMM binds to mitotic spindles and promotes interphase microtubule instability and mitotic spindle integrity (18,19). Uniquely, it is also unconventionally exported onto extracellular surface to partner with CD44, thereby enhancing CD44-mediated tumor progression via ERK1/2 association, and maintaining high proliferative activities and motility of invasive cancer cells (20,21).…”

Centrosomal protein of 192 kDa (Cep192) promotes centrosome-driven spindle assembly by engaging in organelle-specific Aurora A activation

“…Analysis of MT assembly promoted by sperm nuclei, RanGTP, or αAurA beads was carried out as described (27,38,39) with some modifications. The detailed experimental procedures are described in SI Materials and Methods.…”

Kinesin-14 Family Proteins HSET/XCTK2 Control Spindle Length by Cross-Linking and Sliding Microtubules

“…The Ran gradient controls spindle morphogenesis through the regulation of many spindle assembly factors, such as the chromatin-driven MT-nucleating proteins TPX2 (Gruss et al, 2001) and NuSAP (Ribbeck et al, 2006), the spindle pole organizing factor NuMA (Nachury et al, 2001;Wiese et al, 2001), the K-fiber-binding protein HURP (Koffa et al, 2006;Sillje et al, 2006), the mRNA export factor Rae1 (Blower et al, 2005), the tumor suppressor BRCA1 (Joukov et al, 2006), and many others. Because of the complexity of various Ran substrates, we were unable to establish a direct link between Ran regulation and Kinesin-14 function in spindle assembly in somatic cells.…”