Cytoplasmic RING-finger E3 Ub-ligase and sensor of stalled ribosomes; binds to Fpr1p and Smu2p, and triggers K63-linked polyubiquitination of monoubiquitinated 40S subunit Rps3p, from decoding-defective ribosomes, followed by dissociation of the 80S ribosomes and non-functional rRNA decay (NRD) of 18S rRNA; confers rapamycin resistance by competing with rapamycin for Fpr1p binding; accumulates in the nucleus in rapamycin-treated cells; ortholog of D. melanogaster shuttle craft and human NFX1