Alport syndrome (AS) is caused by mutations in type IV collagen alpha 3, alpha 4, and alpha 5 chains. The three chains form a heterotrimer. In this study, we introduced 12 kinds of missense and three kinds of nonsense mutations, corresponding to AS mutations, into the NC1 domain of alpha 5(IV) and characterized the mutant chains. Nine alpha 5(IV) chains with amino acid substitutions and all three truncated alpha 5(IV) chains did not form a heterotrimer and were not secreted from cells. Three alpha 5(IV) chains with amino acid substitutions did, however, form heterotrimers in cells, but these were not secreted from cells. These findings indicate that a defect in heterotrimer formation is the main molecular mechanism underlying the pathogenesis of AS caused by mutation in the NC1 domain. We also showed that even a single amino acid deletion in the carboxyl-terminal region markedly affected the heterotrimerization, indicating that the carboxyl-terminal end is indispensable for heterotrimer formation. (C) 2007 Elsevier Inc. All rights reserved.