PP2A is a major serine/threonine phosphatase in eukaryotic cells which
dephosphorylates many of the proteins involved in signal transduction. Alterations in
PP2A activity have been implicated in a number of diseases including cancer, which has
led to the suggestion that PP2A is a tumor suppressor. PP2A exists in a number of
holoenzyme forms. The core dimer (PP2AD) consists of a structural (PR65) and a
catalytic (PP2Ac) subunit, and can be associated with one of several regulatory B
polypeptides (B, B', B'' and B'''). Interaction with these B subunits probably controls
both the subcellular location and substrate specificity of the phosphatase activity. The
ST antigen of SV40, and ST and MT antigen of the polyoma tumour virus interact with
the PP2A dimer, competing with the B subunits for binding. This interaction has been
shown to be required for the full transforming actions of each virus. In addition to the
trimeric nature of the holoenzyme, it now clear that many more proteins interact with
individual subunits of the PP2A complex, influencing its regulation and activity.
Consequently, to understand how viruses affect the regulation and diverse functions of
PP2A, it is important to study not only its trimeric state, but also its complex
composition. To achieve this, a series of monoclonal antibodies have been generated
against the structural subunit of PP2A and two of the regulatory subunits. The
characterisation of these antibodies and their use in studying PP2A associated proteins
are described in this thesis. In a complementary approach, a novel TAP tagged PR65
subunit has been constructed and used to isolate proteins interacting with PP2A.
Associated proteins have been indentified by MS approaches. Interestingly, the
deubiquitinating enzyme, USP28, has been shown to interact with PP2AD, in immunoprecipitation experiments. This association appears to affect the ubiquitin status of PP2A.