Nuclear Envelope

The nuclear envelope, formed by two membranes with a perinuclear cisternal space between them, separates the nucleoplasm from the cytoplasm

The nuclear envelope is assembled from two (outer and inner) nuclear membranes containing a perinuclear cisternal space between them. The nuclear envelope encloses the chromatin, defines the nuclear compartment, and serves as a membranous boundary between the nucleoplasm and the cytoplasm. The perinuclear clear cisternal space is continuous with the cisternal space of the rER (Fig. 2.58). The two membranes of the envelope are perforated at intervals by nuclear pores that mediate the active transport of proteins, ribonucleoproteins, and RNAs between the nucleus and cytoplasm. The membranes of the nuclear envelope differ in structure and functions:

• The outer nuclear membrane closely resembles the membrane of the endoplasmic reticulum and in fact is contin

b. This electron micrograph, prepared by the quick-freeze deep-etch technique, shows the nucleus, the large spherical object, surrounded by the nuclear envelope. Note that the outer membrane possesses ribosomes and is continuous with the rER. x 12,000. (Courtesy of Dr. John E. Heuser, Washington University School of Medicine.)

uous with rER membrane (see Fig 2.58). Polyribosomes are often attached to ribosomal docking proteins present on the cytoplasmic side of the outer nuclear membrane. • The inner nuclear membrane is supported by a rigid network of protein filaments attached to its inner surface, called the nuclear lamina. In addition, the inner nuclear membrane contains specific lamin receptors and several lamina-associated protein receptors that bind to chromosomes and secure the attachment of the nuclear lamina.

The nuclear lamina lies adjacent to the inner surface of the nuclear envelope, between the membrane and the marginal heterochromatin

The nuclear (fibrous) lamina, a thin, electron-dense protein layer, has a supporting or "nucleoskeletal" function. If the membranous component of the nuclear envelope is disrupted by exposure to detergent, the fibrous lamina remains, and the nucleus retains its shape.

The major components of the lamina, as determined by biochemical isolation, are nuclear lamins, a specialized type of nuclear intermediate filament (see page 53), and lamina-associated proteins (Fig. 2.59). Unlike other cytoplasmic intermediate filaments, lamins disassemble during mitosis and reassemble when mitosis ends. The nuclear lamina appears to serve as a scaffolding for chromatin, chromatin-associated proteins, nuclear pores, and the membranes of the nuclear envelope. In addition, it is involved in nuclear organization, cell cycle regulation, and differentiation. Impairment in nuclear lamina architecture or function is associated with certain genetic diseases and apoptosis.

inner and outer nuclear membrane rough endoplasmic reticulum perinuclear space inner and outer nuclear membrane rough endoplasmic reticulum perinuclear space

0 0

Post a comment