Sld5, A Subunit of the Heterotetrameric GINS Complex is Necessary for Normal Cell Cycle Progression and Genomic Stability

Abstract

Sld5 is one component of the GINS heterotetrameric complex essential to DNA replication. Specifically, GINS is known for its integral role during the G1 to S phase transition in the cell cycle. The GINS complex is comprised of multiple subunits: Psf1, Psf2, Psf3 and Sld5, all of which are highly conserved in eukaryotes. During the initiation of S phase, GINS mediates the association of multiple proteins at replication origins. SLD5 plays a central role in the GINS complex through contact with both Psf1 and Psf2. Due to this pivotal role, Sld5 is the focus of our continuing investigation into the mechanisms of DNA replication and heterochromatin formation in Drosophila. Understanding Sld5 function has employed the use of several approaches. To recognize the range of protein interactions in which SLD5 participates we are using yeast two-hybrid analysis, confirming suspected interactions. In addition to interaction studies we are utilizing two recently identified mutant alleles of SLD5 to understand its function in vivo. These p-element insertion alleles result in the truncation of the Sld5 protein removing a large portion of the C-terminal beta domain in both mutants, a domain that is believed to play a role in facilitating interactions with other proteins. The arrest point determination of Sld5 was completed and shown to occur at the late embryo/early larval stage transition of the developing Drosophila. These homozygous lethal alleles of SLD5 are being used to understand the role of Sld5 in DNA replication through EdU incorporation assays. In addition, possible roles for Sld5 in chromosome biology are being examined. These methods include the analysis of the morphology of chromosomes in polytene tissues, larval brain tissues, and embryos. Roles of Sld5 within the cell cycle have been explored by quantitation of mitotic indexes using larval brain squashes with both alleles of Sld5 showing a marked increase in mitotic figures observed when compared to wild type. In addition, Embryo analysis has revealed severe mitotic defects including asynchrony, cell dropout, and anaphase bridges are presence upon division. Exploration of the Sld5 subunit will further the understanding of the GINS complex and its role in DNA replication, along with its possible roles in chromosome biology and its role in genome maintenance.