Co‐localization of Fyn with CD3 complex, CD45 or CD28 depends on different mechanisms

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Abstract
The Src family protein tyrosine kinase Fyn (p59fyn) plays an important role in thymocyte development and T cell receptor (TCR) signal transduction. Fyn has been shown to associate with the TCR‐CD3 complex, the protein tyrosine phosphatase CD45 and several co‐receptors such as CD28 which are crucial for initiating T cell activation and proliferation. The molecular basis of how Fyn is associated with these transmembrane proteins is largely unknown. To investigate the Fyn association with the TCR‐CD3 complex, CD45 and CD28 at the molecular level, various Fyn/β‐galactosidase fusion proteins were constructed and expressed in Jurkat cells. Co‐localization experiments applying antibody‐induced co‐capping and double immunofluorescence staining techniques were used to study the association of these fusion proteins with the TCR‐CD3 complex, CD45 and CD28. Our results revealed that co‐localization of Fyn with the TCR‐CD3 complex requires the unique N terminus whereas co‐localization with CD45 depends on the unique N terminus, the Src homology (SH)3‐ and a functional SH2 domain. CD28 co‐localizes with Fyn molecules that contain the N terminus and a functional SH2 domain. These results suggest that Fyn association with the TCR‐CD3 complex, CD45 and CD28 is mediated by different molecular mechanisms.