Nuclear localization of p85s6k: functional requirement for entry into S phase

EMBO J. 1994 Apr 1;13(7):1557-65.


Immunolocalization of a newly described isoform of p70s6k, termed p85s6k, demonstrated a predominantly nuclear location in rat embryo fibroblasts (REF-52), a compartment in which growth factor-mediated phosphorylation of S6 has recently been reported. Microinjection of expression vectors encoding either p85s6k or a fusion protein containing only the putative nuclear localization motifs led to the exclusive accumulation of both products in the nucleus. Consistent with such a localization, microinjection of affinity-purified anti-p85s6k IgG into the nucleus, but not the cytoplasm, blocked serum-induced initiation of DNA synthesis. Co-injection into the nucleus of the anti-p85s6k IgG with activated p70s6k, which lacks the antigenic epitope, rescued the S phase block, arguing that the antibody exerts its effects through inhibiting p85s6k function. The results indicate a novel role for S6 phosphorylation in the nucleus distinct from that in the cytoplasm, a role essential for mitogenesis.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Antibodies / pharmacology
  • Cell Compartmentation
  • Cell Nucleus / enzymology*
  • Fibroblasts / cytology
  • Fluorescent Antibody Technique
  • Isoenzymes / immunology
  • Isoenzymes / physiology*
  • Microinjections
  • Molecular Sequence Data
  • Protein Serine-Threonine Kinases / immunology
  • Protein Serine-Threonine Kinases / physiology*
  • Rats
  • Recombinant Proteins
  • Ribosomal Protein S6 Kinases
  • S Phase / drug effects
  • S Phase / physiology*


  • Antibodies
  • Isoenzymes
  • Recombinant Proteins
  • Protein Serine-Threonine Kinases
  • Ribosomal Protein S6 Kinases