Intrinsically disordered proteins are thought to undergo coupled binding and folding upon interaction with their folded partners. In this study, we investigate whether binding of the intrinsically disordered T cell receptor zeta cytoplasmic tail to the well-folded simian immunodeficiency virus Nef core domain is accompanied by a disorder-to-order transition. We show that zeta forms a 1:1 complex with Nef and remains unfolded in the complex. Thus, our findings oppose the generally accepted view of the behavior of intrinsically disordered proteins and provide new evidence of the existence of specific interactions for unfolded protein molecules.