ARHI is a Ras-related small G-protein with a novel N-terminal extension that inhibits growth of ovarian and breast cancers

Robert Z. Luo, Xianjun Fang, Rebecca Marquez, Shu Ying Liu, Gordon B. Mills, Warren S.L. Liao, Yinhua Yu, Robert C. Bast

    Research output: Contribution to journalArticle

    74 Scopus citations

    Abstract

    Our group recently identified Ras homolog member I (ARHI), a novel maternally imprinted tumor suppressor gene that encodes a 26kDa GTP-binding protein with high homology to Ras and Rap. Unlike other Ras family members, ARHI exhibits several unusual structural and functional properties. ARHI contains a unique 34 amino-acid extension at the N-terminus, and differs from Ras in residues critical for GTPase activity and in its putative effector domain. Like Ras, ARHI can bind to GTP with high affinity but has low intrinsic GTPase activity. In addition, while Ras is an oncogene, ARHI functions as an inhibitor for cell growth. 32Phosphorus labeling showed that ARHI is maintained in a constitutively activated GTP-bound state in resting cells, possibly because of impaired GTPase activity. ARHI is associated at the cell membrane through its prenylation at the C-terminal cysteine residue. Mutation of the conserved CAAX box at the C-terminus led to a loss of its membrane association and a decreased ability to inhibit cell growth. Conversion of Ser51 to Asn decreased GTP binding and reduced ARHI's biological activity. Mutation of Ala46 to Val increased the ability of ARHI to inhibit cell growth, associated with a further decrease of its intrinsic GTPase activity. Moreover, conversion of residues in ARHI that are conserved in the Ras family for GTPase activity partially restored the GTPase activity in ARHI. Most strikingly, deletion of ARHI's unique N-terminal extension nearly abolished its inhibitory effect on cell growth, suggesting its importance in ARHI's inhibitory function. Thus, ARHI is a unique Ras family member that retains basic small GTPase function, but exhibits many unusual features. In contrast to most other Ras family members, ARHI has a long N-terminal extension, modest GTPase activity, and constitutive GTP binding in resting cells. Furthermore, unlike the Ras oncogene, ARHI inhibits cell growth, and loss of its expression in cells may contribute to the development of breast and ovarian cancers.

    Original languageEnglish (US)
    Pages (from-to)2897-2909
    Number of pages13
    JournalOncogene
    Volume22
    Issue number19
    DOIs
    StatePublished - May 15 2003

    Keywords

    • ARHI
    • GTP-binding protein
    • GTPase
    • Mutagenesis
    • Protein function
    • Ras superfamily
    • Tumor suppressor gene

    ASJC Scopus subject areas

    • Molecular Biology
    • Genetics
    • Cancer Research

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