FIG. 2.

In vitro kinase assay was carried out as described in Materials and Methods with purified Csk tyrosine kinase (6) and mouse A6/twinfilin. The reaction buffer contained either 10 mM MgCl₂ or 5 mM MnCl₂. The incorporation of ³²P into the substrate, poly(Glu-Tyr), was visualized by an autoradiogram from an SDS–12% polyacrylamide gel. As expected, the Csk tyrosine kinase shows a strong tyrosine kinase activity under both buffer conditions. In contrast, no tyrosine kinase activity can be detected with purified mouse A6/twinfilin in either Mg- or Mn-containing buffers. MW, molecular weight markers.