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preceded the evolution of the Metazoans.
54-56
Proline
986 (Pro
986
- relative position along the triple helix) was
recently identified; this residue is almost 100% hydroxy-
lated (class I site) in A-clade fibrillar collagen includ-
ing a1(I), a1(II) and a2(V), though not in a1(III). Its
modification is reduced or absent in recessive OI due to
CRTAP
,
LEPRE1
or
PPIB
mutations (
Figure 14.4
).
11,13,16,42
Additional proline residues that are N-terminal to the
Pro
986
were identified as partially hydroxylated (class
II sites) in the A-clade and also in the B-clade colla-
gen chains, but within different triple-helical sequence
contexts.
57
These sites do not show the distinct target
sequence of the 986 site and are variably hydroxylated
even in the absence of CRTAP. More recently, the GPP
triplet repeats at the very C-terminal end of the triple
helix were shown to be heavily 3-hydroxylated (class
III sites) but only in a1(I) and a2(I) chains derived from
tendons and not from bone or skin.
58
The modification
of both class II and III sites is not affected by mutations
in
CRTAP
,
LEPRE1
or
PPIB
genes (
Figure 14.4
); interest-
ingly, prolyl 3-hydroxylase 2 which is likely responsible
for the modification of sites on type IV collagen, may
also be involved in the modification of fibrillar collagen
class II sites.
59
Because the relative distance between
each of the class II sites is very close to that of the col-
lagen D-periodicity (234 residues), it has been suggested
that the 3Hyp residues may function in the collagen
intermolecular crosslinking by creation of hydrogen
bonds among the exposed hydroxyl groups during the
process of fibril supramolecular assembly.
57,60
Moreover,
the variability in the amount of class II and III 3Hyp
modifications suggests potential tissue-specific require-
ments of fibrillar collagen deposition and fibril assembly
(e.g., in tendons).
58
While the role of 3Hyp is still poorly
understood, it is also possible that it may ultimately
play a signaling role in the extracellular matrix and
/
or
generate a new binding site for other matrix proteins.
Mutations in
CRTAP
,
LEPRE1
and
PPIB
cause some of
the most severe forms of OI.
11,13,36,42,61
This is likely due
to the fact that the prolyl 3-hydroxylation complex plays
a very early role after procollagen translation in the ER.
Lack of CRTAP causes loss of P3H1 protein and
vice
versa
, while lack of CYPB does not overly affect levels of
CRTAP or P3H1, although some discordant data are pres-
ent in the literature.
13,16,17,52
Loss of CYPB also appears to
cause a less severe reduction of Pro
986
hydroxylation and
a variable amount of collagen overmodification13,51
13,51
while
loss of Pro
986
hydroxylation is always consistently found
with
CRTAP
or
LEPRE1
mutations. The associated over-
modification is more significant in cultured fibroblasts
in vitro
than it is in tissues
in vivo
, and is the result of
slowing of the collagen folding in the ER and the modifi-
cation of additional proline and lysine residues. It is pos-
sible that P3h1 and Crtap play a role in stabilizing CypB
onto the procollagen molecule so that it can execute its
prolyl cis-trans isomerase activity. Recent evidence sug-
gests that CYPB may also be involved with the folding of
the C-terminal propeptide and the association of the pro-
collagen chains which would explain the retarded folding
and also accumulation of unfolded collagen inside the
cell seen with
PPIB
mutations.
52
Additionally, the loss of
Pro
986
hydroxylation could also result in the disruption of
P3H1
CRTAP
CyPB
A4
A3
A2A1
OH
FIGURE 14.4
The prolyl 3-hydroxylation complex and the site of its activity on fibrillar collagens. The complex consists of P3H1, CRTAP
and CyPB, and has 3-hydroxylase, prolyl cis-trans isomerase and collagen chaperone activity. Multiple 3-hydroxyproline residues have now
been identified in different collagen alpha-chains and classified as class I, II and III sites (shown in black (A1), blue (A2, A3, A4) and yellow,
respectively). The prolyl 3-hydroxylation complex only modifies the class I site (A1) which is normally close to 100% hydroxylated.
