Binding of __SP__ __NODE__ protein and mutant __SP__ 1 Nef __SP__ 1 [Nef] protein ( allele nl4 3 ) ( C terminal truncation 207 End N terminal truncation 1 35 ) and mutant __SP__ 1 Nef __SP__ 1 [Nef] protein ( allele nl4 3 ) ( C terminal truncation 36 End ) occurs in a cell fraction from __SP__ .