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the pro-α1(I) chain, and the pro-α1(I) amino-propeptide
region in particular, and suggested that this interaction
is important for chaperoning procollagen processing and
folding.
17
As demonstrated in knockout studies, in the absence
of HSP47, collagen microfibril formation and basement
membrane formation are impaired because of the failure
in the molecular maturation of types I and IV collagens,
respectively.
10
Ishida et al. demonstrated that HSP47
knockout mouse embryos are deficient in the matura-
tion of collagen types I and IV, and collagen triple helices
formed in the absence of HSP47 show increased suscep-
tibility to protease digestion. Fibrils of type I collagen
produced by HSP47−/− cells are abnormally thin and
frequently branched. Type I collagen was highly accu-
mulated in the ER of HSP47−/− cells, and its secretion
rate was much slower than that of HSP47+/+ cells, lead-
ing to accumulation of the insoluble aggregate of type I
collagen within the cells.
18
Transient expression of HSP47
in the HSP47-deficient cells restored normal extracellu-
lar fibril formation and intracellular localization of type
I collagen.
In the ER, HSP47 and collagen prolyl-4-hydroxylase,
which also has chaperone activity, differ from most
other protein chaperones (e.g., calnexin, BiP, GRP94 and
PDI) in that they have a very limited set of clients.
19
As reviewed by Makareeva and Leikin (see Chapter 7)
several chaperone molecules subserve other functions
but also act as procollagen chaperones, e.g., SPARC
(Intracellular Secreted Protein Acidic and Rich in
Cysteine), which binds to the triple-helical domain of
procollagens.
20
Other multifunctional proteins acting as
chaperones are prolyl-3-hydroxylase (P3H1), cartilage-
associated protein (CRTAP) and cyclophilin B (PPIB),
which as noted above complex together to 3-hydroxylate
pro-986. Recessive mutations involving this complex
were recently reported in patients with delayed procol-
lagen folding and moderately severe or lethal OI.
21
Most interestingly, in striking contrast to other chap-
erones, HSP47 preferentially recognizes the folded tri-
ple-helical conformation of its client. Makareeva and
Leikin have provided direct experimental evidence sug-
gesting why such non-traditional chaperone action may
be required for procollagen triple-helix folding. In an
in vitro
study modeling the protein concentration and
temperature conditions of the ER, these authors found
that the human procollagen triple helix spontaneously
folds into its native conformation at 30-34°C but not at
higher temperatures, even in an environment emulat-
ing the ER. Leikina et al. had demonstrated that type I
collagen is unstable at body temperature.
22
General ER
chaperones prevent aggregation and misfolding of pro-
collagen C-propeptide in their traditional role of binding
to unfolded polypeptide chains. However, such binding
only further destabilizes the triple helix. Makareeva and
FIGURE 16.1
HSP47 deficiency alters processing of the type I col-
lagen trimer.
18
Reprinted with permission of the publisher.
HSP47 binds to collagen types I, II and IV in the ER.
HSP47 prevents local unfolding (micro denaturation)
of newly formed triple-helical regions at body tempera-
ture and limits misalignment or improper triple-helix
formation during refolding (
Figure 16.1
).
13
The number
of HSP47-binding sites on any natural collagen has not
yet been determined, but multiple sites have been iden-
tified in collagen chains by studies of synthetic triple-
helical peptides
14,15
as well as CNBr peptides from α
1
(I),
α
2
(I) and α
1
(II) chains.
16
Interestingly, the highest-affinity
sites were found in the N-terminal region of procolla-
gen which may assist in directing to the Golgi once tri-
mer stability has been accomplished.
16
Hu et al. in 1995
demonstrated preferential interaction of HSP47 with
