The Cdk9 and cyclin T subunits of TAK/P-TEFb localize to splicing factor-rich nuclear speckle regions

J Cell Sci. 2001 Apr;114(Pt 8):1491-503. doi: 10.1242/jcs.114.8.1491.


TAK/P-TEFb is an elongation factor for RNA polymerase II-directed transcription that is thought to function by phosphorylating the C-terminal domain of the largest subunit of RNA polymerase II. TAK/P-TEFb is composed of Cdk9 and cyclin T and serves as the cellular cofactor for the human immunodeficiency virus transactivator Tat protein. In this study, we examined the subcellular distribution of Cdk9 and cyclin T1 using high resolution immunofluorescence microscopy and found that Cdk9 and cyclin T1 localized throughout the non-nucleolar nucleoplasm, with increased signal present at numerous foci. Both Cdk9 and cyclin T1 showed only limited colocalization with different phosphorylated forms of RNA polymerase II. However, significant colocalization with antibodies to several splicing factors that identify nuclear 'speckles' was observed for Cdk9 and especially for cyclin T1. The pattern of Cdk9 and cyclin T1 distribution was altered in cells treated with transcription inhibitors. Transient expression of cyclin T1 deletion mutants indicated that a region in the central portion of cyclin T1 is important for accumulation at speckles. Furthermore, cyclin T1 proteins that accumulated at speckles were capable of recruiting Cdk9 and the HIV Tat protein to this compartment in overexpression experiments. These results suggest that cyclin T1 functions to recruit its binding partners to nuclear speckles and raises the possibility that nuclear speckles are a site of TAK/P-TEFb function.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Cell Nucleus / metabolism
  • Cell Nucleus / ultrastructure*
  • Cyclin T
  • Cyclins / analysis*
  • Cyclins / chemistry
  • Cyclins / metabolism*
  • Dactinomycin / administration & dosage
  • Female
  • Gene Products, tat / chemistry
  • Gene Products, tat / metabolism
  • HeLa Cells
  • Humans
  • Nuclear Matrix / ultrastructure*
  • Positive Transcriptional Elongation Factor B
  • Protein Serine-Threonine Kinases / chemistry*
  • Protein Serine-Threonine Kinases / metabolism
  • Protein Subunits
  • RNA Polymerase II
  • Spliceosomes / ultrastructure
  • Subcellular Fractions / metabolism
  • Subcellular Fractions / ultrastructure
  • Transcription, Genetic / drug effects
  • Transcription, Genetic / genetics


  • CCNT1 protein, human
  • Cyclin T
  • Cyclins
  • Gene Products, tat
  • Protein Subunits
  • Dactinomycin
  • Positive Transcriptional Elongation Factor B
  • Protein Serine-Threonine Kinases
  • RNA Polymerase II