The microtubule triplets of the centriole surround an internal lumen. The distal part of the lumen (away from the nucleus) contains a 20-kDa Ca2 -binding protein, centrin (Fig. 2.56). The proximal part of the lumen (close to the nucleus) is lined by -tubulin, which provides the template for the arrangement of the triplet microtubules. In addition, a family of newly discovered  -, -, -, and -tubulin molecules as well as pericentrin protein complexes have also been localized with the centrioles. Other proteins, such a protein p210, form a ring of molecules that appears to link the distal end of the centriole to the plasma membrane. Filamentous connections between the centriole pair have been identified in human lymphocytes. In other organisms, two protein bridges, the proximal and distal connecting fbers, connect each centriole in a pair (see Fig. 2.56). In dividing cells, these connections participate in segregating the centrioles to each daughter cell. In some organisms, the proximal end of
FIGURE 2.54 • Mitotic spindle during normal cell division and in cells lacking centrioles. a. This schematic drawing shows the orientation of the mitotic spindle in a normal cell undergoing mitosis. Note the positions of the centrioles and the distribution of the spindle microtubules. b. In a cell that lacks centrioles, mitosis occurs and a mitotic spindle containing only kinetochore micro-tubules is formed. However, both poles of the mitotic spindle lack astral microtubules, which position the spindle in proper plane during the mitosis. Such a misoriented spindle is referred to as an anastral bipolar spindle. (Based on Marshall WF, Rosenbaum JL. How centrioles work: lessons from green yeast. Curr Opin Cell Biol 2000;12:119–125.) each centriole is attached to the nuclear envelope by contractile proteins called nucleus–basal body connectors (NBBCs). Their function is to link the centriole to the mitotic spindle poles during mitosis. In human cells, the centrosome–nucleus connection appears to be maintained by filamentous structures of cytoskeleton. A distinctive feature of mammalian centrioles is the difference between individual centrioles in the pair. One centriole (termed the mature centriole) contains stalklike satellite processes and sheetlike appendages whose function is not known (see Fig. 2.56). The other centriole (termed the immature centriole) does not possess satellites or appendages.
Centrosome duplication is synchronized with the cell-cycle events and linked to the process of ciliogenesis.
Centrosome dynamics such as duplication or formation of basal bodies for ciliogenesis are synchronized with the cell-
FIGURE 2.55 • Electron micrograph showing parent and daughter centrioles in a fbroblast. Note that the transverse-sectioned centriole in each of the pairs reveals the triplet confguration of microtubules. The lower right centriole represents a mid-longitudinal section, whereas the upper left centriole has also been longitudinally sectioned, but along the plane of its wall.
90,000. (Courtesy of Drs. Manley McGill, D. P. Highfeld, T. M. Monahan, and Bill R. Brinkley.) cycle progresion. Cilia are assembled during the G1 phase; they are most abundant in GO, and are disassembled before cell enters M phase of the cell cycle. These events are depicted in Figure 2.57, which shows an association between centrosome duplication, primary cilium formation, and progression through the cell cycle.