Altered rate of fibronectin matrix assembly by deletion of the first type III repeats

The assembly of fibronectin (FN) into a fibrillar matrix is a complex stepwise process that involves binding to integrin receptors as well as interactions between FN molecules. To follow the progression of matrix formation and determine the stages during which specific domains function, we have developed cell lines that lack an endogenous FN matrix but will form fibrils when provided with exogenous FN. Recombinant FNs (recFN) containing deletions of either the RGD cell-binding sequence (RGD^-) or the first type III repeats (FNΔIII_1-7) including the III₁ FN binding site were generated with the baculovirus insect cell expression system. After addition to cells, recFN matrix assembly was monitored by indirect immunofluorescence and by insolubility in the detergent deoxycholate (DOC). In the absence of any native FN, FNΔIII_1-7 was assembled into fibrils and was converted into DOC-insoluble matrix. This process could be inhibited by the amino- terminal 70 kD fragment of FN, showing that FNΔIII_1-7 follows an assembly pathway similar to FN. The progression of FNΔIII_1-7 assembly differed from native FN in that the recFN became DOC-insoluble more quickly. In contrast, RGD recFNs were not formed into fibrils except when added in combination with native FN. These results show that the RGD sequence is essential for the initiation step but fibrils can form independently of the III_1-7 modules. The altered rate of FNΔIII_1-7 assembly suggests that one function of the missing repeats might be to modulate an early stage of matrix formation.