The Replicative Regulator Protein Geminin on Chromatin in the HeLa Cell Cycle

Abstract

Geminin is believed to have a major function in the regulation of genome replication and cell proliferation. Published evidence shows that geminin specifically interacts with Cdt1 to block its function in the assembly of prereplication complexes. However, in proliferating HeLa cells geminin and Cdt1 are co-expressed during a relatively short time at the G 1 -to-S phase transition. Under these conditions, nearly all Cdt1 and a major part of geminin are bound to chromatin and reside at the same or closely adjacent sites as shown here by chromatin immunoprecipitation. Cdt1 is rapidly degraded early in S phase, but geminin remains bound to the chromatin sites. One function that chromatin-bound geminin could perform is to prevent access to Cdt1 that may escape S phase-dependent degradation or is synthesized in excess. Indeed, Cdt1 continues to be synthesized in HeLa cells in S phase but never accumulates because of the efficient degradation. Therefore, geminin can be eliminated by RNA interference without detectable effects on cell cycle parameters.

Life without geminin

“…18,[31][32][33] Why do different cells show a differential requirement for Geminin for normal cell cycle progression? The critical role of Geminin during the cell cycle is believed to be the inhibition of untimely licensing by restricting the activity of Cdt1.…”

Geminin's Double Life: Chromatin Connections that Regulate Transcription at the Transition from Proliferation to Differentiation

“…However, a fraction of Geminin is found on chromatin. 37,38 In addition, Geminin binds the regulatory elements of the Hoxd11 intron and 3' untranslated region (UTR) and associates with Polycomb repressor complex proteins at these sites. 20 Geminin has also been reported to interact with an HDAC-associated activity, 22 so Gem could potentially repress transcription by recruiting HDACs to Polycomb-regulated loci.…”

“…For example, Geminin depletion in HeLa cells does not cause chromosomal overreplication or cell cycle arrest because Cdt1 levels at S phase are regulated by a secondary redundant mechanism (efficient ubiquitination and proteolysis). 37,[86][87][88] Interestingly, while some interacting proteins bind Geminin in a competitive manner (Cdt1 and Hox/Six3) through overlapping protein domains, some interactions (Gem-Brg1 and Gem-Hox/ Six3/Cdt1 interactions, for example) might occur simultaneously as these involve distinct physical motifs in the Geminin protein. Therefore, Geminin may enable cells to monitor and physically integrate multiple cellular cues and transcriptional events at the transition from proliferation to differentiation.…”

Role of geminin: from normal control of DNA replication to cancer formation and progression?

“…37 Therefore, it has been proposed that geminin becomes necessary as a regulatory factor only when the concentration of Cdt1 remains high during S phase. 38 Geminin ensures Figure 1 Geminin role in DNA replication. During G1 phase, the origin recognition complex (ORC), which constitutively binds to the origins of replication, functions as a substrate for the replication factors Cdc6 and Cdt1.…”

“…It has been shown by Melixian et al, that depletion of geminin does not lead to DNA over-replication in HeLa cells as seen in HCT116, US2OS and normal fibroblast. 31,36,38 Further information is necessary to better delineate the role(s) played by geminin in the regulation of cell cycle machinery in both normal and cancer cells. In our opinion, in transformed cells whose proliferation has escaped the control of important cell cycle regulators such as geminin protein, cell cycle progression may occur independently of what geminin may do to maintain a correct pattern of DNA replication and to control specific mechanisms regulating cell cycle, thus, acting to promote tumor progression rather than as a classic tumor suppressor.…”