Drosophila SUMM4 complex couples insulator function and DNA replication control

Area of Science:

• Molecular Biology
• Genetics
• Chromatin Biology

Background:

• Asynchronous chromosome replication dictates genome function, but mechanisms for early vs. late replication timing are unclear.
• Intercalary heterochromatin replicates late and is underreplicated in polytene chromosomes.
• The SNF2-related factor SUUR causes locus-specific underreplication in Drosophila.

Purpose of the Study:

• To elucidate the mechanism by which SUUR regulates DNA replication.
• To identify SUUR-interacting proteins and characterize their function in replication control.

Main Methods:

• Developed MS-Enabled Rapid protein Complex Identification (MERCI) to isolate protein complexes.
• Biochemical assays to measure SUUR ATPase activity.
• Genetic analysis of SUUR and Mod(Mdg4) mutants in Drosophila.

Main Results:

• Identified the SUMM4 complex, comprising SUUR and Mod(Mdg4)-67.2.
• Mod(Mdg4) stimulates SUUR ATPase activity and is crucial for SUUR localization.
• SUMM4 complex mediates gypsy insulator activity and establishes chromatin barriers.
• Mutations in SUUR or Mod(Mdg4) reverse intercalary heterochromatin underreplication.
• SUMM4 delays replication by attenuating fork progression at euchromatin/heterochromatin boundaries.

Conclusions:

• SUUR, an ATP-dependent motor protein, functions in insulator activity and replication control.
• Chromatin barriers, mediated by SUMM4, can delay DNA replication.
• Architectural proteins play a critical role in establishing replication timing.
• Late replication timing can be established without asynchronous origin firing.