Protein targeting to the nuclear pore. What can we learn from plants?

Characteristic of eukaryotic cells are the numerous types of membrane-bound organelles or compartments found in the cytoplasm, with each type carrying out an essential function for the cell. The spatial separation of proteins and biochemical pathways typical of the various types of organelles requires selective targeting apparatuses. Because each type of organelle contains its own targeting apparatus, proteins destined for a particular organelle must contain the proper targeting signal(s) for entry. These signaldependent targeting pathways ensure that proteins are targeted to the proper organelle. Understanding how proteins are targeted to the different types of organelles is an important goal in the field of cell biology. The nucleus is a double-membrane-bound organelle that separates DNA replication and transcription from protein synthesis. Communication between the nucleus and the cytoplasm is a selective process that occurs through large proteinaceous structures called NPCs, which are embedded in the nuclear envelope. Nucleocytoplasmic communication is bidirectional, and it is essential for the cell to know, for example, when to divide and how to respond to various environmental signals. Proteins involved in cellcycle regulation and transcription factors linked to various signal transduction pathways are examples of proteins that are targeted to the nucleus through NPCs. In addition, the export ribonucleoproteins such as mRNA, tRNA, uridinerich small nuclear RNAs, and rRNA protein complexes are examples of molecules that exit the nucleus through the NPCs. Therefore, understanding the mechanisms that target proteins and ribonucleoproteins into and out of the nucleus is essential to elucidating communication between the nucleus and cytoplasm. Several nuclear import and export targeting signals have been identified and characterized in vertebrates and yeast (for reviews, see Görlich, 1997; Nigg, 1997). Each type of signal is linked to an import and/or export pathway. Signal-mediated import and export are facilitated by a family of carrier proteins called the importin b-like proteins, in conjunction with a small GTPase, Ran/TC4 (for reviews, see Görlich, 1997; Nigg, 1997). The importin b-like proteins function as the nuclear signal receptors for specific import and export pathways; RanGTP regulates the binding of these receptors to the cargo bearing the nucleartargeting signals. The classical NLS is a well-characterized targeting signal found in all eukaryotes, including higher plants (for review, see Hicks and Raikhel, 1995b). Transcription factors and cell-cycle regulators are examples of proteins that contain NLSs. Although there is no consensus sequence for NLSs, they have some common features. Typically, NLSs are rich in basic amino acids and are not cleaved after import. In addition, NLSs are position independent; some proteins can contain multiple NLSs. The NLS import pathway is unique in that a heterodimer of importins b and a is required. In this pathway, the importin a-subunit functions as the receptor modulator during NLS protein import. These import receptors appear to be conserved in all eukaryotes, including higher plants. In plants recent studies have highlighted a number of unusual features, and as our understanding of import in plants increases, we have gained new insights, such as a model for the targeting of proteins from the cytoplasm to the NPC. These advances will contribute to further expansion of our knowledge of nuclear import in eukaryotes.