Ser/Thr phosphorylation of hematopoietic specific protein 1 (HS1) - Implication of protein kinase CK2

Hematopoietic lineage cell-specific protein 1 (HS1), a tyrosine multiphosph orylated protein implicated in receptor-mediated apoptosis and proliferativ e responses, is shown here to become Ser/Thr phosphorylated upon incubation of platelets with radiolabeled inorganic phosphate. The in vivo Ser/Thr ph osphorylation of HS1 is enhanced by okadaic acid and reduced by specific in hibitors of casein kinase (CK)2. In vitro, HS1 is an excellent substrate fo r either CK2 alpha subunit alone (K-m = 47 nm) or CK2 holoenzyme, tested in the presence of polylysine (K-m = 400 nm). Phosphorylation reaches a stoic hiometry of about 2 mol phosphate per mol HS1 and occurs mainly at threonyl residue(s), mostly located in the N-terminal region, but also at seryl res idue(s) residing in the central core of the molecule (208-402), as judged f rom experiments with deleted forms of HS1. Ser/Thr phosphorylation of HS1, either induced in vivo by okadaic acid or c atalysed in vitro by CK2, potentiates subsequent phosphorylation at tyrosyl residues. These data indicate the possibility that regulation of HS1 may a lso be under the control of Ser/Thr phosphorylation, and suggest that in qu iescent cells CK2 could play a role in inducing constitutive Tyr phosphoryl ation of HS1 in the absence of stimuli that activate the protein tyrosine k inase pathway.