Announcements

1
Announcements

• Monday, April 16 the cell cycle, pp. 586-598.
• Wednesday, April 18 protein synthesis, pp.
  684-695.
• Friday, April 20 protein targeting, pp. 695-711.
• Homework problems 19-10, 14 due.

2
Day 39 Outline/Objectives

• Regulation of the cell cycle
• MPF
• Cyclin
• Cell cycle checkpoints
• G2
• Spindle assembly
• G1

• After reading the text, attending lecture, and
  reviewing lecture notes, you should be able to
• Discuss the evidence for and components of
  mitosis-promoting factor.
• Discuss the three main cell cycle checkpoints
  (G1, spindle assembly, and G2) and how passage is
  controlled through the these checkpoints.

3
Regulation of the Cell CycleCell Cycle
Checkpoints
E.g. Oocytes
Differentiating cells
4
Evidence that Cytoplasmic Signals Control the
Cell Cycle
5
Evidence for a Maturation (or Mitosis) Promoting
Factor (MPF) Masui et al. 1971
6
The Discovery of Cyclin

• Tim Hunt (Nobel Prize co-winner, 2001) and MBL
  Physiology class, Woods Hole, MA, in 1982
• Fertilized sea urchin eggs in presence of
  35S-methionine
• Ran SDS-PAGE gel and autoradiography
• Saw the following results

Mitosis 1
Mitosis 2
Cyclin
All other proteins
0 15 30 45 60 75 90 105 120 135
Minutes post-fertilization
7
Synthesis and Degradation of Cyclin correlates
with MPF Activity
8
Cell Division Cycle (cdc) Mutants in Yeast (late
1980s)

• Hartwell (Nobel Prize, 2001) et al., working with
  budding yeast S. cerevisiae
• Found temperature-sensitive mutants stuck in some
  point of cell cycle
• Nurse (Nobel Prize, 2001) et al., working with
  fission yeast S. pombe
• Found gene they called cdc2, essential for
  passing G2 checkpoint
• cdc2 turned out to be a new protein kinase -
  cyclin dependant kinase (Cdk) - with counterparts
  in all eukaryotic cells.

9
G2 Checkpoint Control by MPF

• Active MPF Mitotic Cdk mitotic cyclin
• Cdk is cyclin-dependant kinase
• MPF controls G2 ? M by phosphorylating and
  activating proteins involving in
• Chromosome condensation
• Nuclear envelope breakdown
• Spindle assembly
• Its own self-destruction

10
G2 checkpoint
11
Spindle Assembly Checkpoint Controls Metaphase ?
Anaphase

• MPF () anaphase promoting complex, which
  destroys
• Securin, which allows separin protease to cleave
  cohesin.
• Mitotic cyclin, which causes loss of MPF
  activity, leading to chromosome decondensation
  and envelope reformation.
• Mad2 signal stops coming from kinetochore MTs
  once attached, also () APC.

()
12
G1 checkpoint

• Controlled by G1 Cdk-cyclin
• G1 cyclin levels also vary with the cell cycle
• Many additional levels of phosphorylation,
  dephosphor-ylation regulate.

13
Growth Factor Signaling Through the Ras Pathway ?
crossing of G1 checkpoint
Ras, Raf MAPK cascade Activation of
nuclear TFs Activation of G1 Cdk cyclin genes
G1 ? S
14
Summary Cell Cycle Control

• A clock is running within the cell - of
  synthesis and degradation of cyclins - which
  activate cyclin-dependant kinases (Cdks), which
  activate other proteins to cause checkpoint
  transitions.
• The clock is adjusted (cyclins and Cdks are
  regulated) by other proteins, based on
• External signals (growth factors)
• Internal signals (correct completion of previous
  step)