To determine the role of the extra domain A (EDA) and type III connecting segment (IIICS) of fibronectin in fiber assembly. topographical distribution and proteolytic cleavage, eight full-length human fibronectin cDNA variants (aa0, aa64, aa89, and aa120 variations in the IIICS with or without the EDA) tagged with the V5 epitope were cloned from human endothelial cells and were expressed in CHO-K1 cells. All eight variants were assembled oil cell surfaces. However, only the EDA(+) variants, regardless of the type of the IIICS domain, formed extensive fibrous networks. In contrast, the EDA(-)/aa64 and EDA(-)/aa89 variants were present predominantly as a soluble form. Western analysis of both soluble and cell-associated fibronectin/V5 variants showed that aa64, aa89, and aa120 variants with or without the EDA domain produced the major 50- to 62-kDa C-terminal fragments, whereas the aa0 variants did not, suggesting that the IIICS domain provides proteolytic cleavage sites. (c) 2005 Elsevier Inc. All rights reserved.