The activation of ?/? heterodimeric integrins is the result of highly coordinated rearrangements within both subunits. The molecular interactions between the two subunits, however, remain to be characterized. In this study, we use the integrin ?(L)?(2) to investigate the functional role of the C-linker polypeptide that connects the C-terminal end of the inserted (I) domain with the ?-propeller domain on the ? subunit and is located at the interface with the ?I domain of the ? chain. We demonstrate that shortening of the C-linker by eight or more amino acids results in constitutively active ?(L)?(2) in which the ?I domain is no longer responsive to the regulation by the ?I domain. Despite this intersubunit uncoupling, both I domains remain individually sensitive to intrasubunit conformational changes induced by allosteric modulators. Interestingly, the length and not the sequence of the C-linker appears to be critical for its functionality in ?/? intersubunit communication. Using two monoclonal antibodies (R7.1 and CBR LFA-1/1) we further demonstrate that shortening of the C-linker results in the gradual loss of combinational epitopes that require both the ?I and ?-propeller domains for full reactivity. Taken together, our findings highlight the role of the C-linker as a spring-like element that allows relaxation of the ?I domain in the resting state and controlled tension of the ?I domain during activation, exerted by the ? chain.