Macromolecular component of the subendothelium. Major component of the Descemet's membrane (basement membrane) of corneal endothelial cells. Also a component of the endothelia of blood vessels. Necessary for migration and proliferation of vascular smooth muscle cells and thus, has a potential role in the maintenance of vessel wall integrity and structure, in particular in atherogenesis (By similarity). Homotrimers, or heterotrimers in association with alpha 2(VIII) type collagens. Four homotrimers can form a tetrahedron stabilized by central interacting C-terminal NC1 trimers (By similarity). In the kidney, expressed in mesangial cells, glomerular endothelial cells, and tubular epithelial cells. Proteolytically cleaved by neutrophil elastase, in vitro. Prolines at the third position of the tripeptide repeating unit (G-X-Y) are hydroxylated in some or all of the chains. COL8A1(-)/COL8A2(-) mice exhibit decreased proliferation of mesangial cells, reduced phosphorylation of ERK1/2 and increased p27(KIP1) expression. Diabetic COL8A1(-)/COL8A2(-) mice reveal reduced mesangial expansion and cellularity and extracellular matrix expansion.