Cancer is of paramount medical concern as an increasingly major contributor of disease-related fatalities of significant prevalence –particularly in the context of current statistical/stochastical epidemiological studies which predict that one in three people will contract cancer at some stage of their lives, whilst one in four of these patients will die as a consequence of their particular neoplastic-associated condition.The human Rad9 protein exists in two full-length isoforms (termed Rad9A and Rad9B) whose respective differentially-elevated levels and related expression profiles are distinctive for specific tumour cell tissue types.Most known functions of the DNA damage response protein Rad9 are executed via the well-characterised Rad9-Rad1-Hus1 (“9-1-1”) protein complex, which is loaded onto chromatin in close vicinity to DNA lesion sites.
The chromatin-loaded “9-1-1”complex functions as both a DNA damage “sliding-clamp” sensor and a recruitment platform which modulates and co-ordinates the activities of a wide variety of different proteins implicated in cell cycle checkpoint signalling, steroidal nuclear receptor signalling, protein chaperoning and DNA repair –via associative protein-protein interactions with the C-terminal tail domain of the Rad9 sub-unit.This toroidal, heterotrimeric “9-1-1” DNA sliding-clamp complex is highly conserved and its recently resolved crystal structure shows a functional similarity to the homotrimeric PCNA DNA sliding-clamp complex.
Associative ring formation amongst the individual Rad9, Rad1 and Hus1 sub-units is limited,via stringent steric and thermodynamic parameters, to the heterotrimeric type “9-1-1” DNA sliding-clamp complex configuration.Recent clinical data strongly indicate that over-expression of Rad9, but not Hus1 or Rad1, also promotes tumour growth.Aside from the well-documented phenomena of the modulation of apoptotic signalling and pyrimidine nucleobase biosynthesis activities, comparatively little is known about the “9-1-1”complex-independent functions of the human Rad9 protein –whose dysfunctional activities may be implicated in the development and progression of carcinogenesis.
The initial research emphasis of this Ph.D. project was focused on the elucidation on the mechanism of expression and potential functional roles of a novel N-Terminal truncated (“short”) variant of the Rad9 protein –termed “Rad9-S”, which is expressed in the experimental eukaryotic cell cycle model organism Schizosaccharomyces pombe.Expression of relatively low levels of a constitutive form of Rad9-S were detected in S. pombecells under normal conditions, whilst significantly increased levels of an expressed inducible form of the protein were detected in S. pombe cells as part of an exclusive response to heat shock.In addition to Rad9-S, the constitutive expression of two shorter truncated Rad9 variants –termed Rad9-VSand Rad9-T(“very-short” and “tiny”), was also detected in S. pombe cells.