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. Expressed primarily in the subendothelium of large blood vessels. Also expressed in arterioles and venules in muscle, heart, kidney, spleen, umbilical cord, liver and lung and is also found in connective tissue layers around hair follicles, around nerve bundles in muscle, in the dura of the optic nerve, in cornea and sclera, and in the perichondrium of cartilaginous tissues. In the kidney, expressed in mesangial cells, glomerular endothelial cells, and tubular epithelial cells. Also expressed in mast cells, and in astrocytes during the repair process. Expressed in Descemet's membrane. Some up-regulation in diabetic nephropathy. 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. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry.