Inhibiting proteasome activity causes overreplication of DNA and blocks entry into mitosis in sea urchin embryos

Area of Science:

• Cell Biology
• Molecular Biology
• Developmental Biology

Background:

• The proteasome is known to regulate mitotic cyclin degradation, facilitating exit from mitosis.
• Its role in other cell cycle phases, particularly S phase and G2/M transition, is less understood.

Purpose of the Study:

• To investigate the role of the proteasome in S phase completion and entry into mitosis during the sea urchin embryonic cleavage cycle.
• To determine if proteasome activity is required for the activation of key cell cycle regulators like p34(cdc2) kinase.

Main Methods:

• Utilized structurally related peptide-aldehydes to inhibit proteasome activity in sea urchin embryos.
• Monitored nuclear envelope breakdown, S phase exit, and M phase entry.
• Assessed histone H1 kinase activation and p34(cdc2) tyrosine dephosphorylation.
• Investigated the effect of caffeine on pre-mitotic arrest.

Main Results:

• Proteasome inhibition by peptide-aldehydes blocked nuclear envelope breakdown, correlating with S phase exit and mitosis entry.
• Inhibition of the proteasome prevented mitotic histone H1 kinase activation and p34(cdc2) tyrosine dephosphorylation.
• Arrested embryos showed continued DNA synthesis and enlarged nuclei, indicating S phase progression was not fully halted but entry into mitosis was blocked.
• Caffeine treatment could overcome the pre-mitotic arrest, suggesting involvement of the DNA replication checkpoint.

Conclusions:

• The proteasome is essential for the proper termination of S phase and subsequent initiation of M phase in the early embryonic cell cycle.
• The proteasome plays a critical role in the regulatory pathway leading to embryonic p34(cdc2) kinase activation.
• These findings highlight a broader role for the proteasome in cell cycle control beyond mitotic exit.