Figure 8. AIB1 is phosphorylated by Cdk1/cyclin B but not Cdk1/Cyclin A.

(A) Representative scheme of full length AIB1 and GST fusion fragments generated and named A through E. (B) Qualitative analysis of AIB1 fragments subjected to in vitro phosphorylation with active Cdk1/cyclin B. Reactions were resolved by SDS-PAGE and gel, dried on Whatman paper and exposed for autoradiography. Fragment containing amino acids 792–928 of the Retinoblastoma protein (Rb) was also expressed as a GST fussion protein and 1 µg was used in a parallel reaction as a positive control. (C) Same fragments as in (B) were also incubated with 100 ng of active Cdk1/Cyclin A2 (Cell Signaling). (D) Coomassie staining of the different AIB1 fragments fused to GST. Based in the cualitative amounts of proteins stained, double input of fragment E was used in the radioactive reactions. (E) Autoradiography (upper panel) of AIB1 fragment C (aminoacids 693–933) and fragments harbouring point substitutions serine 728 to alanine (S728A), serine 860 to alanine (S860A) and serine 867 to alanine (S867A), subjected to phosphorylation by complex Cdk1/cyclin B1 in the presence of [γ-³²P]ATP, 1 hour at 30°C. Bottom panel represents a Coomassie staining of the fragments used in the upper kinase reactions. (F) Autoradiography exposure (upper panel) and coomassie staining (lower panel) of AIB1 fragment C and a mutant (C AYA) in which amino acids RYL localized at +11 from serine 728 are mutated to AYA. Fragments were incubated with active Cdk1/cyclin B1 for 1 hour at 30°C in the presence of [γ-³²P]ATP. Reactions were resolved by SDS-PAGE and gel, dried on Whatman paper and exposed for autoradiography. Fragment containing amino acids 792–928 of the Retinoblastoma protein (Rb) was also expressed as a GST fusion protein and used in a parallel reaction as a positive control.