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Role of the ubiquitin-like modifier FAT10 in protein degradation and ...

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Introduction<br />

Chapter 5<br />

Antibodies are <strong>in</strong>valuable tools for <strong>the</strong> study <strong>of</strong> endogenous prote<strong>in</strong>s <strong>and</strong> can be<br />

utilized for example for <strong>the</strong> determ<strong>in</strong>ation <strong>of</strong> tissue specific expression patterns<br />

through western blot or histology or <strong>the</strong> analysis <strong>of</strong> <strong>in</strong>tracellular localization by<br />

immun<strong>of</strong>luorescence. We have previously generated two polyclonal antibodies by<br />

immuniz<strong>in</strong>g rabbits with ei<strong>the</strong>r recomb<strong>in</strong>ant GST-<strong>FAT10</strong> or KLH-coupled syn-<br />

<strong>the</strong>tic peptides (Hipp et al., 2005; Raasi, 2001), which have been successfully used<br />

<strong>in</strong> a number <strong>of</strong> applications such as western blott<strong>in</strong>g, immunoprecipitation <strong>and</strong><br />

immun<strong>of</strong>luorescence. Unfortunately, use <strong>of</strong> <strong>the</strong> GST-<strong>FAT10</strong> antiserum has been<br />

somewhat limited by its high level <strong>of</strong> cross-reactivity <strong>and</strong> we <strong>the</strong>refore decided to<br />

generate monoclonal antibodies directed aga<strong>in</strong>st human <strong>FAT10</strong> through <strong>the</strong> im-<br />

munization <strong>of</strong> mice with recomb<strong>in</strong>ant GST-<strong>FAT10</strong>. Fur<strong>the</strong>rmore, <strong>the</strong> availability<br />

<strong>of</strong> two antibodies derived from different species will enable us to perform a num-<br />

ber <strong>of</strong> additional applications, such as for example <strong>the</strong> detection <strong>of</strong> low levels <strong>of</strong><br />

<strong>FAT10</strong> prote<strong>in</strong> <strong>in</strong> different tissues through <strong>the</strong> comb<strong>in</strong>ation <strong>of</strong> immunoprecipita-<br />

tion <strong>and</strong> western blot.<br />

Results <strong>and</strong> Discussion<br />

S<strong>in</strong>ce a previous attempt to generate <strong>FAT10</strong>-specific monoclonal antibodies <strong>in</strong><br />

BALB/c mice failed to yield any viable hybridomas, we decided to use <strong>FAT10</strong>-<br />

deficient mice for <strong>the</strong> immunization <strong>in</strong> <strong>the</strong> hope that <strong>the</strong>se would carry a higher<br />

number <strong>of</strong> <strong>FAT10</strong>-specific precursor cells. Two mice were immunized with re-<br />

comb<strong>in</strong>ant human GST-<strong>FAT10</strong> <strong>and</strong> after three rounds <strong>of</strong> booster immunizations<br />

displayed a titer high enough to warrant perform<strong>in</strong>g <strong>the</strong> fusion. Unfortunately,<br />

immunizations had to be carried out with a GST-fusion prote<strong>in</strong> as proteolytic re-<br />

moval <strong>of</strong> GST resulted <strong>in</strong> <strong>the</strong> immediate precipitation <strong>of</strong> untagged <strong>FAT10</strong>. Sim-<br />

ilarly, expression <strong>of</strong> <strong>FAT10</strong> with a smaller tag (e.g. His6) did not yield any sol-<br />

uble prote<strong>in</strong> ei<strong>the</strong>r. To exclude those hybridomas which produced antibodies di-<br />

rected aga<strong>in</strong>st GST, hybridoma supernatants were screened by ELISA for reactiv-<br />

ity aga<strong>in</strong>st both GST-<strong>FAT10</strong> as well as GST alone. The <strong>in</strong>itial screen<strong>in</strong>g yielded<br />

five positive clones, two <strong>of</strong> which could be stabilized after two (<strong>in</strong> <strong>the</strong> case <strong>of</strong> 4F1)<br />

or three (<strong>in</strong> <strong>the</strong> case <strong>of</strong> 3A11) rounds <strong>of</strong> subclon<strong>in</strong>g. Isotyp<strong>in</strong>g by ELISA revealed<br />

both antibodies to be <strong>of</strong> <strong>the</strong> IgG2a isotype.<br />

112

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