The Golgi apparatus consists of disc-shaped smooth cisternae that are assembled in stacks (dic-tyosomes), having a diameter of approximately 1 p,m and associated with numerous small membrane-bound vesicles (Figure 1-1-4).
Figure 1-1-4. Cytoplasm
The Golgi apparatus has two distinct faces:
■ The cis (forming) face is associated with the RER.
• The trans (maturing) face is often oriented toward the plasma membrane. The transmost region is a network of tubular structures known as the trans-Golgi network (TGN) (Figure 1-1-5).
Clinical Correlate l-Cell Disease
Phosphorylation of mannose in glycoproteins targets proteins to lysosomes. Phosphate is added in a two-step sequence of reactions that are catalyzed by W-acetylglucosamine-phosphotransferase and A/-acetylglucosaminidases.
A deficiency in A/-a cety (gl u cosam i n e-p h os p hotra nsferase results in l-cell disease (mucolipidosis II), in which a whole family of enzymes is sent to the wrong destination. It is characterized by huge inclusion bodies in cells caused by the accumulation of undegraded glycoconjugates in lysosomes missing the hydrolases that normally degrade these macromolecules. The missing enzymes are found in the plasma and other body fluids, where they have normal levels of activity. The absence of the mannose-6-phosphate on the hydrolases results in their secretion rather than their incorporation into lysosomes.
The disease results in skeletal abnormalities, coarse features, restricted joint movements, and psychomotor retardation. Symptoms are generally noted at birth, and the life span is less than 10 years.
A somewhat less severe form of the disease with a later onset and potential survival into adulthood is ailed pseudo-Hurler polydystrophy.
There is no treatment for either disease, but prenatal diagnosis is available.
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