Abstract
Fibronectin (FN) is an extracellular matrix protein that is secreted by many cell types and binds predominantly to the cell surface receptor Integrin α5β1. Integrin α5β1 binding initiates the step-wise assembly of FN into fibrils, a process called fibrillogenesis. We and several others have demonstrated critical effects of fibrillogenesis on cell migration and metastasis. While immunostaining and microscopy methods help visualize FN incorporation into fibrils, with each fibril being at least 3 μm in length, the first study that developed a method to biochemically fractionate FN to quantify fibril incorporated FN was published by Jean Schwarzbauer's group in 1996. Our protocol was adapted from the original publication, and has been tested on multiple cell types including as shown here in MCF10A mammary epithelial and Caki-1 renal cancer epithelial cells. Using two detergent extractions, cellular FN is separated into detergent insoluble or fibril incorporated FN and soluble FN or unincorporated fractions. To determine whether fibrillogenesis utilizes a recycled pool of FN, we have used a Biotin labeled FN (FN-Biotin) recycling assay, that has been modified from a previous study. Using a combination of the recycling assay and deoxycholate fractionation methods, one can quantitatively demonstrate the extent of fibrillogenesis in cells under different experimental conditions and determine the source of FN for fibrillogenesis.