Nearly all brain tumors develop following the progressive accumulation of genetic alterations of oncogenes and tumor suppressor genes (such as p53 and retinoblastoma protein). Furthermore, aberrations in the nuclear matrix often contribute to genomic instabilities and the development of cancer. We have previously shown that nuclear-restricted protein/brain (NRP/B), a member of the BTB/Kelch repeat family, is a nuclear matrix protein normally expressed in neurons but not in astrocytes, and that it is an early and specific marker of neurons during the development of the central nervous system. Here, we show aberrant expression of NRP/B in human brain tissues. NRP/B is expressed in the cytoplasm of human brain tumor cells (glioblastoma, GBM) arising from astrocytes. NRP/B mutations (13 mutations in the Kelch domains, two in the intervening sequence (IVS) domain and two in the BTB domain) were detected in brain tumor cell lines (A-172, CCF-STTG1, SK-N-SH and U87-MG) and in primary human malignant GBM tissues (eight samples). More importantly, we found that NRP/B mutants, but not wild-type (wt) NRP/B, increased the activation of ERK and consequently promoted cell proliferation, attenuated caspase activation and suppressed the cellular apoptosis induced by the stressful stimulus cisplatin (10 microM). These events were observed to occur via a p53-mediated pathway. In addition, while wt NRP/B was associated with actin, mutations in the Kelch domains of NRP/B led to its reduced binding affinity to actin. Thus, alterations and gene mutations within the NRP/B gene may contribute to brain tumorigenesis by promoting cell proliferation, suppressing apoptosis and by affecting nuclear cytoskeleton dynamics.