Roles of the nuclear envelope are considered in the regulation of nuclear protein import, ribonucleoprotein export, and coupling of DNA replication to the cell cycle. First, evidence is discussed that indicates that neutral and acidic amino acids can be important in nuclear localization signals as well as the widely acknowledged basic amino acids. Second, the recognition of nuclear localization signals by their receptor "importin" is discussed, focusing on the different roles of the two subunits of importin. Third, a role for the subunit of importin in RNP export is considered together with the question of how the direction of traffic through nuclear pores is determined. The final part of this article considers evidence that the nuclear membrane prevents reinitiation of DNA replication in Xenopus eggs, by excluding a "licensing factor" that is essential for DNA replication. Replication licensing in Xenopus appears to involve several proteins including the MCM (minichromosome maintenance) complex and ORC, the origin recognition complex, which must bind before the MCM complex can bind to chromatin.
EXPERIMENTAL CELL RESEARCH 229
204 - 211 (1996)
ARTICLE NO. 0361
Copyright © 1996 Academic Press, Inc.
1Address (1996): Zentrum für
Molekulare Biologie, Universität Heidelberg, im Neuenheimer
Feld 282, D-69120 Heidelberg, Germany.
2Address (1996): Joint Diseases Laboratory, Shriners Hospital for Crippled Children, 1529 Cedar Avenue, Montreal, Quebec, Canada H3G 1A6.