Treatment of cultured human skin fibroblasts with increasing doses of y-interferon produces a distinct reduction of steady-state levels of the a3 chain of collagen VI mRNA by about 60% but not of the a1 and a2 chain mRNAs. A similar decrease was also observed for collagen I and 111 mRNA while fibronectin mRNA remained at the same level. The decrease in a3(VI) mRNA is accompanied by a reduced synthesis of collagen VI and by a reduced deposition of both collagen VI and fibronectin in urea-insoluble form in the cell matrix. No other y-interferon effects were observed for fibronectin biosynthesis. Immunoprecipitation of metabolically labeled collagen VI demonstrated a strongly reduced synthesis (by 65 -80Y0) of intracellular a3(VI) chains with no decrease found for al(V1) and a2(VI) chains. All three chains were, however, found to be reduced in the culture medium. Pepsin treatment of immunoprecipitated collagen V1 showed similar chain ratios for material in the culture medium obtained in the absence or presence of y-interferon. It indicates that correctly assembled heterotrimers of the composition [al(VI) a2(VI) a3(VI)] are formed and secreted also in the absence of an equivalent a3(VI) chain synthesis but at a reduced rate. The data support previous predictions from sequence analyses J . Biol. Chem. 263, 18601-186061 that collagen VI molecules composed of all three constituent chains are more stable than other assembly alternatives.Several minor collagen types which are deposited in the extracellular matrix besides the major fiber-forming collagens 1, I1 and I11 and fibronectin have been recently identified [I]. They include collagen VI which is a structurally unique component found in the connective tissue of almost every organ [2]. Its molecular structure is distinguished from other collagens by a particularly short triple helix with large globular domains on both ends. These molecules have the potential to assemble into dimers and tetramers which form large microfibrillar aggregates stabilized by disulfide bonds [3]. Immunoelectron microscopy has shown that such fibrils represent a major part of the microfibrillar network within the extracellular matrix the composition of collagen VI is controlled at the transcriptional level. These three mRNAs have been recently cloned [14] and were shown to be distinctly different in their sequences which encode the triple-helical domain of the protein [16]. The distribution of cysteines and Gly-Xaa-Yaa triplet imperfections led also to the prediction that only a heterotrimer assembly of the chains would yield collagen VI molecules with the potential to form fully stable oligomers. This was of some interest since non-coordinated regulation of the gene expression either of a3(VI) or of al(V1) plus a2(VI) chains was observed for skin fibroblasts when grown in a collagen gel [15] and for cornea fibroblasts and some tumor cells [14]. The pattern of regulation could reflect the different localization of human collagen VI genes to either chromosome 2 (a3) or chromosome 21 (al, a2) ...