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Nucleus

All nucleated cells

''Lamin C is a splice product of lamin A.

''Lamin C is a splice product of lamin A.

teins form the attachment plaques for intermediate filaments, an essential part of desmosomes and hemidesmo-somes. The interaction of intermediate filaments with cell-to-cell and cell-to-extracellular matrix junctions provides mechanical strength and resistance to extracellular forces. Table 2.2 summarizes the characteristics of the three types of cytoskeletal filaments.

Centrioles and Microtubule-Organizing Centers

Centrioles represent the focal point around which the MTOC assembles

Centrioles, visible in the light microscope, are paired, short, rod-like cytoplasmic cylinders built from nine microtubule triplets. In resting cells, centrioles have an orthogonal orientation: one centriole in the pair is arrayed at right angles to the other. Centrioles are usually found in close proximity to the nucleus, often partially surrounded by the Golgi apparatus, and associated with a zone of amorphous, dense pericentriolar material. The region of the cell containing the centrioles and pericentriolar material is called the MTOC or centrosome (Fig. 2.48). The

MTOC is the region where most microtubules are formed and from which they are then directed to specific destinations within the cell. Therefore, the MTOC controls the number, polarity, direction, orientation, and organization of microtubules formed during interphase of the cell cycle. Development of the MTOC itself depends solely on the presence of centrioles. When centrioles are missing, the MTOCs disappear, and formation of microtubules is severely impaired.

The MTOC contains centrioles and numerous ring-shaped structures that initiate microtubule formation

The MTOC contains an amorphous matrix of more than 200 proteins, including y-tubulins that are organized in ring-shaped structures. Each y-tubulin ring serves as the starting point (nucleation site) for the growth of one microtubule that is assembled from tubulin dimers; a- and (3-tubulin dimers are added with specific orientation to the y-tubulin ring. The minus end of the microtubule remains attached to the MTOC, and the plus end represents the growing end directed toward the plasma membrane (see Fig. 2.48).

table 2.2. Summary Characteristics of Three Types of Cytoskeletal Elements

Actin Filaments (Microfilaments) Intermediate Filaments

Microtubules

Microtubules

Shape

Diameter (nm) Basic protein subunit

Enzymatic activity Polarity

Assembly process

Source of energy required for assembly

Characteristics

Associated proteins

Location in ceil

Major functions

Double-stranded linear helical array Rope-like fibers

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