Biology 457 & 557 - Week 8
as proof that degradation of cyclin B is required for exit from mitosis, a mutant
mRNA encoding cyclin B that lacked the destruction box was added to RNase
treated egg extracts
MPF activity increased and mitotic structures formed
thus a fall in MPF activity and exit from mitosis depends on
the exact signal that triggers degradation of cyclin B during anaphase is not yet
known
when MPF protein kinase activity reaches a high level, it phosphorylates
polyubiquitination of cyclin B occurs
since cyclin B is an essential subunit of MPF, its degradation causes
another important function of MPF is to phosphorylate specific serine residues in
nuclear lamins
this phophorylation causes depolymerization of the lamin intermediate
filaments
this is reversible and can get repolymerization of lamins
lastly, MPF or those protein kinases activated by MPF-catalyzed phosphorylation,
are thought to phosphorylate microtubule-associated proteins
all these different types of phosphorylations act together to control
later in mitosis,
cellular dynamics return to interphase conditions triggered by
evidence that anaphase may be induced by polyubiquitination
when add a synthetic polypeptide containing the destruction box of cyclin B to
untreated egg extract
the added extra destruction-box peptide is thought to act as a substrate for the
polyubiquitination system
this implies that polyubiquitination and subsequent degradation of proteins other
than cyclin B is required
from what you know about anaphase, what type of substrates may be targeted?
also found that removal of phosphates from nuclear lamins coincides with lamin
repolymerization
when MPF is inactivated by the degradation of cyclin B at the onset of anaphase
thus, these studies of MPF in Xenopus egg extracts demonstrates that MPF
protein kinase activity induces the onset of mitosis
just like that mentioned for Xenopus, mammalian cells also have cyclin-dependent protein kinases (Cdks)
most studies on mammalian cell-cycle control have been done with cultured cells
binding of these polypeptide growth factors to cell-surface receptors initiates a signal-transduction cascade
mammalian cells cultured in the absence of growth factors are arrested with a diploid complement of chromosomes in a period of the cell cycle known as
if growth factors are added to the culture medium the cells can pass through this
once cells have passed this restriction point, they are committed to enter the
what regulates passage of mammalian cells through the cell cycle?
mammalian cells use a small group of related
named Cdk1, 2 ,3, 4, 5 in order of discovery
(Cdk 3 is present in low levels and its function remains unclear)
several human cyclins have also been identified including cyclins D (1, 2, 3) and E
cyclins A and B
human cyclins D and E are able to complement defective yeast Cln proteins and
perform S phase promoting functions which indicates the critical substrates of G1
Cdk-cyclin complexes are conserved between humans and yeast
these conserved substrates probably include
homologous cyclin A and B proteins have been found in
entry into S phase
when mammalian cells are stimulated with growth factors
cyclin D and its mRNA are unstable and cyclin D levels fall dramatically
in contrast, when cells are constantly cycling because of constant exposure to growth factors
the function of the Cdk-cyclin D complex is to drive mammalian cells through
like cyclins D and E, Cdk2 and Cdk4 are not expressed in G0 cells
when growth factors are removed, the Cdk2 and 4 protein levels fall
the next important step is the induction of entry into S phase
the main complex controlling this step is
this complex is the protein kinase principally responsible
experiments have been done in which Cdk2 was removed
soon after mammalian cells enter S phase
synthesis of cyclin A begins as cells approach the G1-S transition
late in DNA synthesis, cyclin B (MPF) levels rise
Cdk1 also associates with cyclin A
release of blockage at this step is accomplished when cells
there is an induction of transcription of multiple genes
early – induced within minutes
what type of genes are induced first?
activation of cellular transcription factors such as
the identification of these regulated, cellular forms of transcription
factors came from the discovery that
when viral forms of these proteins
form heterodimers with each other and with other leucine zipper proteins
induction of early response genes is not blocked by inhibitors of protein
synthesis…..Why?
after peaking at about 30 min after addition of growth factors, the
concentrations of the early response mRNAs fall to a lower level
this drop in transcription of early response genes is blocked by inhibitors
of protein synthesis……What does this indicate about some of the early
response proteins?
additionally, these early response genes induce transcription of delayed response genes
What phase of the cell cycle do D-type cyclins help control? Cyclin E?