Fission yeast _p107^wee1 mitotic inhibitor is a tyrosine/serine kinase

Abstract

THE fission yeast ^wee1+ gene product is a dose-dependent, negative regulator of entry into mitosis^1,2. ^wee1+ encodes a protein of relative molecular mass 107,000 (M_r 107K), the C-terminal third of which has strong similarities with the serine/threonine protein kinase family^2,3. Here we report that _p107^wee1 immune complexes phosphorylate _p107^wee1 equally on serine and tyrosine residues, and also phosphorylate an exogenous substrate, angiotensin II, on tyrosine. Both kinase activities are attributable to _p107^wee1 because they are also observed when weel ⁺ is expressed in heterologous systems; both are abolished by a point mutation in the ATP-binding domain, and both behave like an asymmetric monomer of M_r 114K on gel filtration and density-gradient centrifugation. Thus the ^wee1+ gene product is representative of a novel class of protein kinase that phosphorylates both serine and tyrosine residues.