In a cell free system , binding of mutant __NODE__ protein ( N terminal truncation 1 639 ) and __SP__ Pgc 1alpha [__NODE__] protein is greater than binding of mutant __NODE__ protein ( N terminal truncation 1 639 ) and mutant __SP__ Pgc 1alpha [__NODE__] protein ( with its LXXLL motif 1 3 mutated ) .