EFFECT OF THE MUTATION OF TYROSINE-713 IN P93C-FES ON ITS CATALYTIC ACTIVITY AND ABILITY TO PROMOTE MYELOID DIFFERENTIATION IN K562 CELLS

The protooncogense protein-tyrosine kinase c-fes plays an active role in the induction of terminal myeloid differentiation in myeloid leukemia cells. Although p93c-fes contains two autophosphorylation sites, it is not known what role they play in its catalytic or biological activities. To address this question, the major autophosphorylation site at tyrosine 713 was mutated to phenylalanine (YF713), and the mutated cDNA was expressed in a baculovirus system to assess catalytic activity, as well as in an inducible retrovirus to determine its biological activity. The major phosphopeptide in p93c-fes in vitro contained Y713 and was absent in the YF713 mutant, which exhibited an 85% loss of autophosphorylation activity. The catalytic activity of p93c-fesYF713 With either RCM-lysozyme or poly(Glu,Tyr)4:1 as substrate was reduced by 85 and 78%, respectively, in comparison to p93c-fes. Retroviral infection of K562-cells with the c-fes cDNA under the control of the mouse metallothionein promoter increased superoxide formation, phagocytosis, CD13 and CD33 antigen expression, and doubling time 4-6 days after induction. Cells infected with c-fesYF713 exhibited 40% less superoxide formation but similar levels of phagocytosis, CD13/CD33 antigen, and doubling time in comparison to cells infected with c-fes. The level of phosphotyrosine-containing proteins did not markedly differ between K562 cells expressing either neo, c-fes, or c-fesYF713, with the exception of a reduction in the level of a 210-kDa protein specifically in both c-fes-expressing cell lines. The p210 was tentatively identified as bcr-abl, whose level was also reduced in cells expressing c-fes or c-fesYF713. These results indicate that loss of the major autophosphorylation site at Y713 in p93c-fes attenuates some, but not all, properties of differentiated myeloid cells. In addition, the differentiated phenotype induced by c-fes is associated with a reduction in bcr-abl, which may be associated with induction of differentiation and suppression of oncogenesis.