Human Homolog

GANP-PCID2-DSS1 represents the human homolog of the yeast Sac3-Thp1-Sem1 complex. GANP is the region homologous to Sac3, while Thp1 correlates to PCID2 and Sem1 correlates to DSS1. These human and yeast homologs exhibit the same protein-protein interactions within their respective complexes and the same overall function.

The crystal structure of the PCID2-DSS1 complex was elucidated at a 2.1-Å-resolution, after growing the crystals at 292K by hanging drop vapor diffusion.

As illustrated in the pull-down assays in Figure 1, the GANP-PCID2-DSS1 complex assembles in a similar way to its yeast homolog.

Figure 1. SDS-PAGE pull-down assays, stained with Coomassie blue, determine the overall assembly of the complex. This was monitored in vitro.

Like the interaction between Thp1 and Sem1, PCID2 is insoluble without the presence of DSS1, as seen in lanes 1 and 2. Furthermore, like the interaction between Sac 3 and Sem1, GANP solubility is not affected by co-expression of DSS1; it is soluble when expressed on its own (lane 3) and this expression does not alter in the presence of DSS1 (lane 4). In effect, lanes 3-5 reveal that DSS1 is not pulled down unless PCID2 is present. Hence, PCID2 and DSS1 must be associated to enable the formation of the GANP-PCID2-DSS1 complex. This is because DSS1 retains the stabilization properties of Sem1 on PCID2. Indicating that this stabilization is a conserved function.

Furthermore, from the crystal structure it was revealed that PCID2 has a C-terminal winged helix domain, like Thp1, and that DSS1 binds to PCID2 via its C-terminal helix in a similar fashion to the way Sem1 binds to Thp1. This is illustrated in Figure 2.

Figure 2. The interaction between the winged helix domain of PCID2, shown in cyan, and the C-terminal domain of DSS1, shown in yellow.

Overall, this determines a strongly conserved structural homology and interaction interface between the yeast and human homologs of this complex.

- Written by Tamara Casteels