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 a s interactions between FN molecules. To follow the progression of matr ix formation and determine the stages during which specific domains fu nction, we have developed cell lines that lack an endogenous FN matrix but will form fibrils when provided with exogenous FN, Recombinant FN s (recFN) containing deletions of either the RGD cell-binding sequence (RGD(-)) or the first type III repeats (FN Delta III1-7) including th e 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 d etergent deoxycholate (DOG). In the absence of any native FN, FN Delta III1-7 was assembled into fibrils and was converted into DOG-insolubl e matrix. This process could be inhibited by the amino-terminal 70 kD fragment of FN, showing that FN Delta III1-7 follows an assembly pathw ay similar to FN. The progression of FN Delta III1-7 assembly differed from native FN in that the recFN became DOG-insoluble more quickly. I n contrast, RGD(-) recFNs were not formed into fibrils except when add ed in combination with native FN. These results show that the RGD sequ ence is essential for the initiation step but fibrils can form indepen dently of the III1-7 modules. The altered rate of FN Delta III1-7 asse mbly suggests that one function of the missing repeats might be to mod ulate an early stage of matrix formation.