plex was also associated with the replication fork in Xenopus laevis egg extracts, and a large molecular machine, containing Cdc45, GINS, and MCM2-7, was proposed as the unwindosome to separate the DNA strands at the replication fork. , GINS must be a critical factor for not only the initiation process, but also the elongation process in eukaryotic DNA replication. S. solfataricus GINS interacts with MCM and primase, suggesting that GINS is involved in the replisome. The concrete function of GINS in the replisome remains to be determined. No stimulation or inhibition of either the helicase or primase activity was observed by the interaction with S. solfataricus GINS in vitro . On the other hand, the DNA helicase activity of P. furiosus MCM is clearly stimulated by the addition of the P. furiosus GINS complex, as described above. contrast to S. solfataricus and P. furiosus , which each express a Gins23 and Gins51, Thermoplasma acidophilum has a single Gins homolog, Gins51. The recombinant Gins51 protein from T. acidophilum was confirmed to form a homotetramer by gel filtration and electron microscopy analyses. Furthermore, a physical interaction between T. acidophilum Gins51 and Mcm was detected by a surface plasmon resonance analysis (SPR). Although the T. acidophilum Gins51 did not affect the helicase activity of its cognate MCM, when the equal ratio of each molecule was tested in vitro , an excess amount of Gins51 clearly stimulated the helicase activity (Ogino et al., unpublished). In the case of T. kodakarensis, the ATPase and helicase activities of MCM1 and MCM3 were clearly stimulated by T. kodakarensis GINS in vitro . It is interesting that the helicase activity of MCM1 was stimulated more than that of MCM3. interactions between the T. kodakarensis Gins and Mcm proteins were also detected. These reports suggested that the MCM-GINS complex is a common part of the replicative helicase in Archaea (Figure 3). Recently, the crystal structure of the T. kodakarensis GINS tetramer, composed of Gins51 and Gins23 was determined, and the structure was conserved with the reported human GINS structures. Each subunit of human GINS shares a similar fold, and assembles into the heterotetramer of a unique trapezoidal shape. Sld5 and Psf1 possess the?-Helical (A) domain at the N-terminus and the?-Stranded domain (B) at the C-terminus (AB-type). the other hand, Psf2 and Psf3 are the permuted version (BA-type). The backbone structure of each subunit and the tetrameric assembly of T. kodakarensis GINS are similar to those of human GINS. However, the location of the C-terminal B domain of Gins51 is remarkably different between the two GINS structures. homology model of the homotetrameric GINS from T. acidophilum was performed using the T. kodakarensis GINS crystal structure as a template. The Gins 51 protein has a long disordered region inserted between the A and B domains and this allows the conformat...
