Abstract
PCNA is a ring-shaped protein that encircles DNA, providing a platform for the association of a wide variety of DNA-processing enzymes that utilize the PCNA sliding clamp to maintain proximity to their DNA substrates. PCNA is a homotrimer in eukaryotes, but a heterotrimer in crenarchaea such as Sulfolobus solfataricus. The three proteins are SsoPCNA1 (249 residues), SsoPCNA2 (245 residues) and SsoPCNA3 (259 residues). The heterotrimeric protein crystallizes in space group P2(1), with unit-cell parameters a = 44.8, b = 78.8, c = 125.6 A, beta = 100.5 degrees. The crystal structure of this heterotrimeric PCNA molecule has been solved using molecular replacement. The resulting structure to 2.3 A sheds light on the differential stabilities of the interactions observed between the three subunits and the specificity of individual subunits for partner proteins.
Highlights
PCNA is a trimeric ring-shaped protein that encircles DNA
Partner proteins interact with PCNA via a PIP-box peptide that makes contact with the interdomain-connecting loop (IDCL) of PCNA (Fig. 1) and up to three different proteins could potentially be loaded onto a single PCNA trimer simultaneously, suggesting that PCNA can act as a molecular ‘tool-belt’
The structure consists of three monomers, SsoPCNA1, SsoPCNA2 and SsoPCNA3, which share only $22% sequence identity (Table 2)
Summary
PCNA (proliferating cell nuclear antigen) is a trimeric ring-shaped protein that encircles DNA. Whilst most archaea encode a homotrimeric PCNA molecule like the eukaryotic version, the crenarchaeote Sulfolobus solfataricus and other Sulfolobus species possess a heterotrimeric PCNA (SsoPCNA; Dionne et al, 2003). This increased complexity allows the opportunity for each subunit to evolve selectivity for binding partners and this has been shown to be the case (Dionne et al, 2003; Dionne & Bell, 2005; Roberts et al, 2003). We report the crystal structure of the PCNA heterotrimer from S. solfataricus to 2.3 A
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