SummaryLaminin-binding integrins (a3b1, a6b1, a6b4, a7b1) are almost always expressed together with tetraspanin CD151. In every coexpressing cell analyzed to date, CD151 makes a fundamental contribution to integrin-dependent motility, invasion, morphology, adhesion and/or signaling. However, there has been minimal mechanistic insight into how CD151 affects integrin functions. In MDA-MB-231 mammary cells, tetraspanin CD151 knockdown impairs a6 integrin clustering and functions without decreasing a6 integrin expression or activation. Furthermore, CD151 knockdown minimally affects the magnitude of a6 integrin diffusion, as measured using single particle tracking. Instead, CD151 knockdown has a novel and unexpected dysregulating effect on the mode of a6 integrin diffusion. In control cells a6 integrin shows mostly random-confined diffusion (RCD) and some directed motion (DMO). In sharp contrast, in CD151-knockdown cells a6 integrin shows mostly DMO. In control cells a6 diffusion mode is sensitive to actin disruption, talin knockdown and phorbol ester stimulation. By contrast, CD151 knockdown cell a6 integrin is sensitive to actin disruption but desensitized to talin knockdown or phorbol ester stimulation, indicating dysregulation. Both phorbol ester and EGF stimulate cell spreading and promote a6 RCD in control cells. By contrast, CD151-ablated cells retain EGF effects but lose phorbol-ester-stimulated spreading and a6 RCD. For a6 integrins, physical association with CD151 promotes a6 RCD, in support of a6-mediated cable formation and adhesion. By comparison, for integrins not associated with CD151 (e.g. av integrins), CD151 affects neither diffusion mode nor av function. Hence, CD151 support of a6 RCD is specific and functionally relevant, and probably underlies diverse CD151 functions in skin, kidney and cancer cells.