Integrin receptors connect the extracellular matrix to the cell cytoskeleton to 
provide essential forces and signals. To examine the contributions of the β1 
integrin cytoplasmic tail to adhesive forces, we generated cell lines expressing 
wild-type and tail mutant β1 integrins in β1-null fibroblasts. Deletion of β1 
significantly reduced cell spreading, focal adhesion assembly, and adhesive 
forces, and expression of human β1 (hβ1) integrin in these cells restored 
adhesive functions. Cells expressing a truncated tail mutant had impaired 
spreading, fewer and smaller focal adhesions, reduced integrin binding to 
fibronectin, and lower adhesion strength and traction forces compared to 
hβ1-expressing cells. All these metrics were equivalent to those for β1-null 
cells, demonstrating that the β1 tail is essential to these adhesive functions. 
Expression of the constitutively-active D759A hβ1 mutant restored many of these 
adhesive functions in β1-null cells, although with important differences when 
compared to wild-type β1. Even though there were no differences in 
integrin-fibronectin binding and adhesion strength between hβ1- and 
hβ1-D759A-expressing cells, hβ1-D759A-expressing cells assembled more but 
smaller adhesions than hβ1-expressing cells. Importantly, hβ1-D759A-expressing 
cells generated lower traction forces compared to hβ1-expressing cells. These 
differences between hβ1- and hβ1-D759A-expressing cells suggest that regulation 
of integrin activation is important for fine-tuning cell spreading, focal 
adhesion assembly, and traction force generation.