Cyclins A and B1 in the Human Cell Cycle

  1. Joan Marsh
  1. Jonathon Pines and
  2. Tony Hunter

Published Online: 28 SEP 2007

DOI: 10.1002/9780470514320.ch12

Ciba Foundation Symposium 170 - Regulation of the Eukaryotic Cell Cycle

Ciba Foundation Symposium 170 - Regulation of the Eukaryotic Cell Cycle

How to Cite

Pines, J. and Hunter, T. (2007) Cyclins A and B1 in the Human Cell Cycle, in Ciba Foundation Symposium 170 - Regulation of the Eukaryotic Cell Cycle (ed J. Marsh), John Wiley & Sons, Ltd., Chichester, UK. doi: 10.1002/9780470514320.ch12

Author Information

  1. Molecular Biology and Virology Laboratory, The Salk Institute for Biological Studies, P. O. Box 85800, San Diego, CA 92186-5800, USA

  1. Wellcome/CRC Institute, Tennis Court Road, Cambridge CB2 1QR, UK

Publication History

  1. Published Online: 28 SEP 2007

ISBN Information

Print ISBN: 9780471934462

Online ISBN: 9780470514320



  • cyclins;
  • human cell cycle;
  • mitosis;
  • serine/threonine kinase;
  • transcription factor


Cyclins are a family of proteins involved in the regulation of the eukaryotic cell cycle. The first cyclins to be isolated were the A- and B-type cyclins and we have been studying their behaviour in human somatic cells. The levels of both cyclin A and B1 are regulated transcriptionally as well as post-translationally; both are rapidly and specifically degraded in mitosis. Cyclin A synthesis commences at the start of S phase and the protein is predominantly nuclear, whereas cyclin B1 appears during S phase and is primarily cytoplasmic. Cyclin B1 moves into the nucleus just at the start of mitosis and associates with condensed chromosomes and the mitotic spindle. Both cyclin A and cyclin B1 bind to and activate a protein serinekhreonine kinase subunit; cyclin A associates with p33cdk2 and with p34cdc2, whereas cyclin B1 seems to bind exclusively to p34cdc2. Cyclin A-associated kinase activity appears much earlier in the cell cycle than that of cyclin B1, which appears only at the G2 to M transition. Therefore cyclin A may play a role in the events of S phase as well as G2 and M phases. Cyclin A forms a cell cycle-dependent complex with p33cdk2 and the transcription factor E2F, although the function of this complex is not yet clear. We conclude that cyclins A and B1 may differentially regulate the cell cycle in several ways. They form complexes with distinct protein kinases and these complexes are active at different times in the cell cycle; they form distinct multiprotein complexes, such as with the transcription factor E2F; and they are localized to different parts of the cell where different substrates will be available to them.