Threonine Phosphorylation Sites in the β2 and β7 Leukocyte Integrin Polypeptides

Abstract

The cytoplasmic domains of integrins play a key role in a variety of integrin-mediated events including adhesion, migration, and signaling. The molecular mechanisms that enhance integrin function are still incompletely understood. Because protein kinases are known to be involved in the signaling and the activation of integrins, the role of phosphorylation has been studied by several groups. The β2 leukocyte integrin subunit has previously been shown to become phosphorylated in leukocytes on cytoplasmic serine and functionally important threonine residues. We have now mapped the phosphorylated threonine residues in activated T cells. After phorbol ester stimulation, all three threonine residues (758–760) of the threonine triplet became phosphorylated but only two at a time. CD3 stimulation leads to a strong threonine phosphorylation of the β2 integrin, but differed from phorbol ester activation in that phosphorylation occurred only on threonine 758. The other leukocyte-specific integrin, β7, has also been shown to need the cytoplasmic domain and leukocyte-specific signal transduction elements for integrin activation. Cell activation with phorbol ester, and interestingly, through the TCR-CD3 complex, caused β7 integrin binding to VCAM-1. Additionally, cell activation led to increased phosphorylation of the β7 subunit, and phosphoamino acid analysis revealed that threonine residues became phosphorylated after cell activation. Sequence analysis by manual radiosequencing by Edman degradation established that threonine phosphorylation occurred in the same threonine triplet as in β2 phosphorylation.