Chapter 19. Regulation of Cyclin B

_feedback: Northern blots could be utilized as a first step to evaluate whether cyclin B expression is regulated at the transcriptional level. Western blots could be utilized in a similar fashion to evaluate the role of cyclin B translation. In both cases, cellular extracts could be isolated at various points in the cell cycle and subsequently analyzed using autoradiography in the case of a Northern blot or labelled antibodies in the case of a Western blot.

_feedback: Yes—cyclin B could be regulated at the post-translational level through phosphorylation at specific amino acid sites. In addition, cyclin B activity could be regulated through other post-translational modifications including glycosylation, carboxylation, methylation or acylation. Moreover, the activity of cyclin B could be regulated through localization or compartmentalization within the cell. Signal-transduction mechanisms based on the cell’s ability to react to its external environment could play a role in these events.

_feedback: Cyclin B is a mitotic cyclin. Its expression could be mediated by events in the cell’s external environment that promote cellular proliferation, for example, in the presence of growth hormones. In these cases, the cell would have to recognize that DNA replication has already taken place. One potential manner by which this could be regulated is through expression of specific markers on the cell surface that signal that S phase is complete and that M phase is about to commence. Signal-transduction mechanisms could be regulated by external peptides or other moieties that interact either directly or indirectly with these specific G₂ phase cell-surface markers. Such signal-transduction mechanisms could promote either the synthesis or activity of cyclin B.