Type IV collagen is a heterotrimeric molecule, which contains the N-terminal 7S, a central triple-helical domain, and the globular C-terminal NC1 domain. A zipper-like mechanism of triple helix formation, starting from the C-terminus, has been proposed for most collagens but for collagen type IV there has only been indirect evidence so far. In this study we expressed trimeric human collagen type IV to compare the effects of different structural variants on the formation of collagen IV molecules. Our data show that the NC1 but not 7S domain is essential for the chain association and initiation of triple helix formation. This strongly suggests an N-to-C terminal mechanism of triple helix formation. Additionally, we could show that the human alpha2(IV) chain can form chimeric alpha1.alpha1.alpha2(IV) heterotrimers with mouse subunits when expressed in PF-HR9 cells. (C) 2004 Elsevier Inc. All rights reserved.