title: Masking of a nuclear signal motif by monoubiquitination leads to mislocalization and degradation of the regulatory enzyme cytidylyltransferase.
abstract: Monoubiquitination aids in the nuclear export and entrance of proteins into the lysosomal degradative pathway, although the mechanisms are unknown. Cytidylyltransferase (CCTalpha) is a proteolytically sensitive lipogenic enzyme containing an NH(2)-terminal nuclear localization signal (NLS). We show here that CCTalpha is monoubiquitinated at a molecular site (K(57)) juxtaposed near its NLS, resulting in disruption of its interaction with importin-alpha, nuclear exclusion, and subsequent degradation within the lysosome. Cellular expression of a CCTalpha-ubiquitin fusion protein that mimics the monoubiquitinated enzyme resulted in cytoplasmic retention. A CCTalpha K(57R) mutant exhibited an extended half-life, was retained in the nucleus, and displayed proteolytic resistance. Importantly, by using CCTalpha-ubiquitin hybrid constructs that vary in the intermolecular distance between ubiquitin and the NLS, we show that CCTalpha monoubiquitination masks its NLS, resulting in cytoplasmic retention. These results unravel a unique molecular mechanism whereby monoubiquitination governs the trafficking and life span of a critical regulatory enzyme in vivo.
authors.affiliation: University of Iowa, Department of Biochemistry, Roy J. and Lucille A. Carver College of Medicine, Iowa City, IA 52242, USA.
authors.forename: Bill B
authors.initials: BB
authors.lastname: Chen
authors.forename: Rama K
authors.initials: RK
authors.lastname: Mallampalli