J. Fitzgerald et al., The N-terminal N5 subdomain of the alpha 3(VI) chain is important for collagen VI microfibril formation, J BIOL CHEM, 276(1), 2001, pp. 187-193
Collagen VI assembly is unique within the collagen superfamily in that the
alpha1(VI), alpha2(VI), and alpha3(VI) chains associate intracellularly to
form triple hlelical monomers, and then dimers and tetramers, which are sec
reted from the cell. Secreted tetramers associate end-to-end to form the di
stinctive extracellular microfibrils that are found in virtually all connec
tive tissues. Although the precise protein interactions involved in this pr
ocess are unknown, the N-terminal globular regions, which are composed of m
ultiple copies elf von Willebrand factor type A-like domains, are likely to
play a critical role in microfibril formation, because they are exposed at
both ends of the tetramers. To explore the role of these subdomains in col
lagen VI intracellular and extracellular assembly, alpha3(VI) cDNA expressi
on constructs with sequential N-terminal deletions were stably transfected
into SaOS-2 cells, producing cell lines that express alpha3(VI) chains with
N-terminal globular domains containing modules N9-N1, N6-N1, N5-N1, M4-N1,
N3-N1, or N1, as well as the complete triple helix and C-terminal globular
domain (C1-C5). All of these transfected alpha3(VI) chains were able to as
sociate with endogenous (alpha1(VI) and alpha2(VI) to form collagen VI mono
mers, dimers, and tetramers, which were secreted. Importantly, cells that e
xpressed alpha3(VI) chains containing the N5 subdomain, alpha3(VI) N9-C5, N
6-C5, and N5-C5, formed microfibrils and deposited a collagen VI matrix. In
contrast, cells that expressed the shorter alpha3(Vl) chains, N4-C5, N3-C5
, and N1-C5, were severely compromised in their ability to form end-to-end
tetramer assemblies and failed to deposit a collagen VI matrix. These data
demonstrate that the alpha3(VI) N5 module is critical for microfibril forma
tion, thus identifying a functional role for a specific type A subdomain in
collagen VI assembly.