L. Valmu et al., Characterization of beta(2) (CD18) integrin phosphorylation in phorbol ester-activated T lymphocytes, BIOCHEM J, 339, 1999, pp. 119-125
Integrins are transmembrane proteins involved in cell-cell and cell-extrace
llular-matrix interactions. The affinity and avidity of integrins for their
ligands change in response to cytoplasmic signals. This 'inside-out' activ
ation has been reported to occur also with beta(2) integrins (CD18). The be
ta(2) integrin subunit has previously been shown to become phosphorylated i
n T lymphocytes on cytoplasmic serine and the functionally important threon
ine residues after treatment with phorbol esters or on triggering of T-cell
receptors. We have now characterized the phosphorylation of beta(2) integr
ins in T-cells in more detail. When T-cells were activated by phorbol ester
s the phosphorylation was mainly on Ser(756). After inhibition of serine/th
reonine phosphatases, phosphorylation was also found in two of the threonin
e residues in the threonine triplet 758-760 of the beta(2) cytoplasmic doma
in. Activation of T-cells by phorbol esters resulted in phosphorylation in
only approx. 10%, of the integrin molecules. Okadaic acid increased this ph
osphorylation to approx. 30% of the beta(2) molecules, assuming three phosp
horylation sites. This indicates that a strong dynamic phosphorylation exis
ts in serine and threonine residues of the beta(2) integrins.