Connective Tissue Answers 183of proline and lysine. The process is both co- and posttranslational and,
therefore, occurs during, or more often after, the amino acids are inserted
into nascent collagen polypeptide chains in the RER. Those two amino
acids are characteristic of collagen. Hydroxyproline, which constitutes 10%
of collagen, is often used to determine the collagen content of various tis-
sues. Hydroxylation of proline stabilizes the triple helix through interchain
hydrogen bonds, and hydroxylation of lysine is critical for the cross-linking
stage of collagen assembly.
91.The answer is c.(Alberts, pp 1098–1100. Junqueira, pp 106–110.)Non-
helical registration peptides at the ends of the triple helix prevent tropocol-
lagen assembly in the RER, Golgi apparatus, and secretory vesicles. Collagen
is synthesized as pro-α–chains, which are assembled into procollagen mol-
ecules (triple helix) in the rough endoplasmic reticulum. Procollagen is sub-
sequently transported in transfer vesicles to the Golgi for packaging into
secretory vesicles. Transport of secretory vesicles is an energy- and micro-
tubule-dependent process. Outside of the cell, N-terminal and C-terminal
specific procollagen peptidases cleave the nonhelical registration peptides,
which results in the formation of tropocollagen. Tropocollagen sponta-
neously assembles in a staggered array to form collagen fibrils. Lysyl oxi-
dase(answer a)is an extracellular enzyme responsible for the formation of
covalent cross-links between tropocollagen molecules. Fibrils form colla-
gen fibers under the influence of other extracellular matrix constituents,
such as proteoglycans and glycoproteins. Collagenases (answer d)specifi-
cally cleave tropocollagen in the extracellular matrix.
92.The answer is a.(Alberts, pp 1099, 1107–1108.)The primary function
of entactin (nidogen) is to cross-link laminin to type IV collagen. The basal
lamina is formed by interactions between type IV collagen, laminin,
entactin, and the proteoglycan perlecan. Integrins like laminin receptors
(answer b)bind cells to the basal lamina; fibronectin receptors bind cells
to the extracellular matrix (answer c).Laminin receptors in the cell mem-
brane also organize the assembly of the basal lamina. Collagen (answer d)
is cross-linked by covalent intramolecular and intermolecular cross-links
that form primarily between the nonhelical segments at the ends of the
collagen molecules. Lysyl oxidase is a key enzyme in the cross-linking
process; it deaminates lysine and hydroxylysine to form aldehyde groups
that react with each other to form the covalent bonds. Actin is cross-linked