IDENTIFICATION OF A BINDING-SITE IN C-ABL TYROSINE KINASE FOR THE C-TERMINAL REPEATED DOMAIN OF RNA-POLYMERASE-II

The c-abl proto-oncogene encodes a nuclear tyrosine kinase that can ph osphorylate the mammalian RNA polymerase II (RNAP II) on its C-termina l repeated domain (CTD) in vitro. Phosphorylation of the CTD has previ ously been shown to require the tyrosine kinase and the SH2 domain of Abl. We show here that a CTD-interacting domain (CTD-LD) at the C-term inal region of c-Abl is also required. Deletion of the CTD-ID causes t he K-m (0.4 mu M) to increase by 2 orders of magnitude. Direct binding of the CTD-ID to CTD and to RNAP II could be demonstrated in vitro. P hosphorylation of a recombinant glutathione S-transferase-CTD by c-Abl was observed in cotransfected COS cells. Mutant Abl proteins lacking the CTD-ID, while capable of autophosphorylation, neither phosphorylat ed nor associated,vith the glutathione S-transferase-CTD in vivo. Tran sient overexpression of c-Abl also led to a four- to fivefold increase in the phosphotyrosine content of the RNAP II large subunit. Moreover , the large subunit of RNAP II could be coprecipitated with c-Abl. Tyr osine phosphorylation of the coprecipitated RNAP II was again dependen t on the presence of the CTD-ID in Abl. Finally, the ability of c-Abl to phosphorylate and associate with RNAP II could be correlated with t he enhancement of transcription by c-Abl in transient cotransfection a ssays. Taken together, these observations demonstrate that c-Abl can f unction as a CTD kinase in vitro as well as in vivo.