A short amino acid sequence able to specify nuclear location

An SV40-adenovirus 7 hybrid virus, PARA(cT), has been described that is defective for the nuclear transport of SV40 large tumor antigen. An SV40(cT) mutant was constructed using SV40 early and late region DNA fragments derived from PARA(cT) and wild-type SV40 respectively. The SV40(cT)-3 construct is defective for viral replication, but can be propagated in COS-1 cells. T antigen induced by SV40(cT)-3 is localized in the cytoplasm of infected cells. The cT mutation also inhibits the transport of wild-type T antigen; COS-1 cells lose their constitutive expression of nuclear T antigen after infection with SV40(cT)-3. Sequence analysis revealed that the cT mutation results in the replacement of a positively charged lysine in wild-type T antigen with a neutral asparagine at amino acid number 128, demonstrating that the alteration of a single amino acid is sufficient to abolish nuclear transport. Implications of the cT mutation on possible mechanisms for the transport of proteins to the nucleus are discussed.

Nucleoplasmin is the most abundant protein of the nucleus of Xenopus laevis oocytes. It rapidly enters the nucleus after being injected into oocyte cytoplasm. Partial proteolysis of the nucleoplasmin pentamer reveals two structural domains within each subunit: a relatively exposed "tail" and a protected "core." When all the "tails" have been removed from the pentamer the residual "core" remains pentameric. This pentameric core fails to enter the nucleus, remaining stably in the cytoplasm. A single tail region attached to the pentamer is sufficient to transport it into the nucleus. The rate of accumulation in the nucleus, but not its final extent, depends on the number of tails per pentamer. The detached (monomeric) tails rapidly accumulate in the oocyte nucleus, indicating that the tail structure is sufficient for selective accumulation. Pentameric cores diffuse throughout the nucleus but are retained when microinjected into the nucleus, indicating that the tail is necessary for entry but not for retention within the nucleus. An improved method for purification of nucleoplasmin is also described.