title: Regulation of p73 by Hck through kinase-dependent and independent mechanisms.
abstract: BACKGROUND
  p73, a p53 family member is a transcription factor that plays a role in cell cycle, differentiation and apoptosis. p73 is regulated through post translational modifications and protein interactions. c-Abl is the only known tyrosine kinase that phosphorylates and activates p73. Here we have analyzed the role of Src family kinases, which are involved in diverse signaling pathways, in regulating p73.
  
  
  RESULTS
  Exogenously expressed as well as cellular Hck and p73 interact in vivo. In vitro binding assays show that SH3 domain of Hck interacts with p73. Co-expression of p73 with Hck or c-Src in mammalian cells resulted in tyrosine phosphorylation of p73. Using site directed mutational analysis, we determined that Tyr-28 was the major site of phosphorylation by Hck and c-Src, unlike c-Abl which phosphorylates Tyr-99. In a kinase dependent manner, Hck co-expression resulted in stabilization of p73 protein in the cytoplasm. Activation of Hck in HL-60 cells resulted in tyrosine phosphorylation of endogenous p73. Both exogenous and endogenous Hck localize to the nuclear as well as cytoplasmic compartment, just as does p73. Ectopically expressed Hck repressed the transcriptional activity of p73 as determined by promoter assays and semi-quantitative RT-PCR analysis of the p73 target, Ipaf and MDM2. SH3 domain- dependent function of Hck was required for its effect on p73 activity, which was also reflected in its ability to inhibit p73-mediated apoptosis. We also show that Hck interacts with Yes associated protein (YAP), a transcriptional co-activator of p73, and shRNA mediated knockdown of YAP protein reduces p73 induced Ipaf promoter activation.
  
  
  CONCLUSIONS
  We have identified p73 as a novel substrate and interacting partner of Hck and show that it regulates p73 through mechanisms that are dependent on either catalytic activity or protein interaction domains. Hck-SH3 domain-mediated interactions play an important role in the inhibition of p73-dependent transcriptional activation of a target gene, Ipaf, as well as apoptosis.
authors.affiliation: Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad, India. preeti@ccmb.res.in <preeti@ccmb.res.in>
authors.forename: Preeti
authors.initials: P
authors.lastname: Paliwal
authors.forename: Vegesna
authors.initials: V
authors.lastname: Radha
authors.forename: Ghanshyam
authors.initials: G
authors.lastname: Swarup
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism; D019556:COS Cells; D018797:Cell Cycle Proteins; D002522:Chlorocebus aethiops; D002945:Cisplatin / Q000494:pharmacology; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D015870:Gene Expression / Q000187:drug effects; D018922:HL-60 Cells; D006367:HeLa Cells; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D019000:Phosphotyrosine / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding / Q000187:drug effects; D021381:Protein Transport / Q000187:drug effects; D051572:Proto-Oncogene Proteins c-hck / Q000737:chemistry / Q000378:metabolism*; D013816:Thermodynamics; D014157:Transcription Factors / Q000378:metabolism; D015533:Transcriptional Activation / Q000187:drug effects / Q000235:genetics; D000072160:Tumor Protein p73; D025521:Tumor Suppressor Proteins / Q000378:metabolism*; D018909:src Homology Domains; D019061:src-Family Kinases / Q000378:metabolism*
chemical_list: D051017:Apoptosis Regulatory Proteins; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; C000606460:TP73 protein, human; D014157:Transcription Factors; D000072160:Tumor Protein p73; D025521:Tumor Suppressor Proteins; C456659:YY1AP1 protein, human; D019000:Phosphotyrosine; C496631:HCK protein, human; D051572:Proto-Oncogene Proteins c-hck; D019061:src-Family Kinases; D002945:Cisplatin