Transport if loaded

Transport if loaded uclear transporters can be viewed as taxis that move cargo across the nuclear envelope. On page 649, Plafker et al. report that importin-11, a nuclear transport receptor , may be an especially selective cabbie. Importin-11 transports UbcM2, a ubiquitin (Ub)-conjugating enzyme, but it does so only when the enzyme is charged with a Ub at its active site. UbcM2 is an E2 enzyme, which works with E1 and E3 proteins to polyubiquitinate and tag proteins for degradation in the proteasome. To test whether importin-11 preferentially transports the Ub-charged UbcM2 or the unloaded enzyme, Plafker et al. performed coimmunoprecipitation assays with wild-type UbcM2, a mutant enzyme that is constitutively loaded with Ub, or a mutant that cannot be loaded. Importin-11 selectively bound the Ub-charged forms of UbcM2. Furthermore , in vitro pull-down assays showed that, if ATP or the E1 enzyme that loads Ub onto the UbcM2 active site were depleted, importin-11 did not bind UbcM2. In cell assays, catalytically inactive UbcM2 failed to localize to the nucleus. Only a subset of E2 enzymes bound to importin-11. This specific interaction may control the enzyme's access to potential substrates, including some involved in cell cycle progression. Plus, if importin-11 gobbles up all the Ub-charged UbcM2, then the enzyme cannot ubiquitinate cytoplasmic proteins. N ES cells without teratomas herapeutic use of embryonic stem cells may be hampered by their proclivity to form pluripotent tumors called teratomas. On page 723, Bieberich et al. describe the use of a ceramide analogue, S18, to induce apoptosis in a subpopulation of embryoid body–derived stem cells (EBCs). Cells that survive the treatment express the neural marker nestin and differentiate into neural progenitors when injected into the brains of young mice. They do not form teratomas. The S18 selectively affects those EBCs that express prostate apoptosis response-4 (PAR-4) protein, an endogenous inhibitor of atypical PKC ␨. Significantly, the majority of the PAR-4–expressing cells also express Oct-4, a marker for pluripotency. Almost all Oct-4 ϩ cells were positive for PAR-4, suggesting that teratoma formation might be prevented via elimination of PAR-4-expressing cells with ceramide-induced apoptosis before injection into animals. Sure enough, after injection into the brains of mice, treated cells differentiated into neural cells and some benign tumors, T but no invasive tumors. Untreated cells gave rise to a significant number of terato-mas in the same animals. Bieberich et al. hypothesize that coexpression of Oct-4 and PAR-4 …