Notably, one subunit of APC itself must remain phosphorylated at a single amino acid during sister chromatid separation, and this phosphate group must be removed by PP2A to allow exit from anaphase. Do you think this phosphorylated amino acid is a serine or a threonine? Explain.
Notably, one subunit of APC itself must remain phosphorylated at a single amino acid during sister chromatid separation, and this phosphate group must be removed by PP2A to allow exit from anaphase. Do you think this phosphorylated amino acid is a serine or a threonine? Explain.
Human Anatomy & Physiology (11th Edition)
11th Edition
ISBN:9780134580999
Author:Elaine N. Marieb, Katja N. Hoehn
Publisher:Elaine N. Marieb, Katja N. Hoehn
Chapter1: The Human Body: An Orientation
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Transcribed Image Text:7. Progression from metaphase to anaphase in the cell cycle, and subsequent exit from anaphase,
requires sequential events involving the Anaphase Promoting Complex (APC). APC is a multi-
protein complex that has ubiquitin ligase activity (see pp. 611-612 in your textbook re: ubiquitin).
When APC adds adds ubiquitin to a substrate protein, the protein is targeted for degradation by the
proteasome. Two important substrates of APC are the proteins securin and cyclin B. Destruction
of securin allows the separation of sister chromatids at the beginning of anaphase, whereas
subsequent destruction of cyclin B allows cells to exit anaphase (see Figure 24-18 in your
textbook).
APC
Cdh-1
Cdc20-
Securin
Mitotic cyclins
Cohesin
G1
Metaphase
Anaphase
For APC to bind specific substrates, it requires unique adaptor proteins that help it to recognize its
substrates. The adaptor protein Cdc20 allows APC to recognize and ubiquitinate securin, while the adaptor
Cdh-1 allows APC to recognize and ubiquitinate cyclin B (see figure above). At the beginning of mitosis,
the adaptor proteins are each phosphorylated at three sites. The phosphates on the adaptor proteins must be
removed in order for the adaptors to interact with APC. The protein phosphatase PP2A, which is present
during mitosis, can dephosphorylate both Cdc20 and Cdh-1. It has been proposed that differential kinetics
of dephosphorylation of serines and threonines by PP2A controls APC targeting and sequential events
during anaphase. Thus a study was carried out to examine the enzymatic activity of PP2A on two substrate
polypeptides that were identical with the exception that one contained phosphoserine (pS) as the PP2A
substrate, and the second contained phoshothreonine (pT) as the PP2A substrate. The substrate amino acid
sequences, and the kinetic properties of PP2A acting on these polypeptide substrates, are presented in the
table.
Kat (min')
Polypeptide Sequence
WDVSPYSL(pS)PVSNKSQ
WDVSPYSL(pT)PVSNKSQ
Km (uM)
286
400
35
880
Transcribed Image Text:B) Notably, one subunit of APC itself must remain phosphorylated at a single amino acid during
sister chromatid separation, and this phosphate group must be removed by PP2A to allow exit
from anaphase. Do you think this phosphorylated amino acid is a serine or a threonine?
Explain.
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