Proteins containing nuclear localization signals are actively transported into the nucleus through NPCs, while proteins containing nuclear export signals are transported out of the nucleus to the cytosol. Some proteins, including the nuclear of the nuclear lamina, which initiates the nuclear envelope to break up. Dephosphorylation of the lamins reverses the process. an analogous phosphorylation and dephosphorylation cycle occurs for some nucleoporins and proteins of the inner nuclear membrane, and some of these dephosphorylations are also involved in the reassembly process. as indicated, the nuclear envelope initially re-forms around individual decondensing daughter chromosomes. eventually, as decondensation progresses, these structures fuse to form a single complete nucleus.
mitotic breakdown of the nuclear envelope occurs in all metazoan cells. however, in many other species, such as yeasts, the nuclear envelope remains intact during mitosis, and the nucleus divides by fission.
import and export receptors, continually shuttle between the cytosol and nucleus. The monomeric GTPase Ran provides both the free energy and the directionality for nuclear transport. Cells regulate the transport of nuclear proteins and RNA molecules through the NPCs by controlling the access of these molecules to the transport machinery. Newly transcribed messenger RNA and ribosomal RNA are exported from the nucleus as parts of large ribonucleoprotein complexes. Because nuclear localization signals are not removed, nuclear proteins can be imported repeatedly, as is required each time that the nucleus reassembles after mitosis.
The TransporT of proTeins inTo miToChonDria anD ChloroplasTs
Mitochondria and chloroplasts (a specialized form of plastids in green algae and plant cells) are double-membrane-enclosed organelles. They specialize in ATP synthesis, using energy derived from electron transport and oxidative phosphorylation in mitochondria and from photosynthesis in chloroplasts (discussed in Chapter 14). Although both organelles contain their own DNA, ribosomes, and other components required for protein synthesis, most of their proteins are encoded in the cell nucleus and imported from the cytosol. Each imported protein must reach the particular organelle subcompartment in which it functions.
There are different subcompartments in mitochondria (Figure 12–19A): the internal matrix space and the intermembrane space, which is continuous with the cristae space. These compartments are formed by the two concentric mitochondrial membranes: the inner membrane, which encloses the matrix space and forms extensive invaginations called cristae, and the outer membrane, which is in contact with the cytosol. Protein complexes provide boundaries at the junctions where the cristae invaginate and divide the inner membrane into two domains: one inner membrane domain surrounds the cristae space, and the other domain abuts the outer membrane. Chloroplasts also have an outer and inner membrane, which enclose an intermembrane space, and the stroma, which is the chloroplast equivalent of the mitochondrial matrix space (Figure 12–19B). They have an additional subcompartment, the thylakoid space, which is surrounded by the thylakoid membrane. The thylakoid membrane derives from the inner membrane during plastid development and is pinched off to become discontinuous with it. Each of the subcompartments in mitochondria and chloroplasts contains a distinct set of proteins.
New mitochondria and chloroplasts are produced by the growth of preexisting organelles, followed by fission (discussed in Chapter 14). The growth depends mainly on the import of proteins from the cytosol. The imported proteins must be transported across a number of membranes in succession and end up in the appropriate place. The process of protein movement across membranes is called protein translocation. This section explains how it occurs.