The question of how specific proteins are accumulated in the cell nucleus, while other proteins are excluded, is of fundamental importance as an example of the general phenomenon of intracellular protein targeting. Nuclear protein import is required for some types of signal transduction (the communication of extracellular signals to organelles within the cell, including the nucleus) and thus bears on how cell growth is regulated. Therefore, nuclear protein import is relevant to the study of diseases, such as cancer, involving aberrant control of cell proliferation. The mechanisms of nuclear import are not well understood. However, it is known that signal peptides within the amino-acid sequence of nuclear proteins can target proteins to the cell nucleus. Recent experiments by the principal investigator have shown the existence of a signal-sequence recognition event in nuclear import in which nuclear proteins become bound to the nuclear pore. This binding, likely mediated by a signal-sequence receptor, is ATP-independent, whereas the subsequent translocation of the protein across the pore requires ATP. The signal receptor has not yet been identified, nor have the proteins involved in translocation across the nuclear pore. The work proposed here is concerned with identifying the macromolecules involved in nuclear import and elucidating their roles in the mechanism. The studies will employ a recently developed in vitro system, based on an extract of Xenopus eggs, that functions in authentic nuclear import. Using this system, a factor in the extract has now been shown to be required for nuclear protein import.
The first aim of the research will be to purify this factor and characterize it with regard to its composition, intracellular location, and function in the transport mechanism. Secondly, the proposed studies will determine whether receptors are heterogeneous in their signal-recognition repertoire; this will involve competition studies with synthetic signal peptides derived from sequences of several different nuclear proteins.
