The design of artificial retroviral restriction factors

In addition to the ability to bind the retroviral capsid protein, the retroviral restriction factors Fv1, Trim5α and Trim5–CypA share the common property of containing sequences that promote self-association. Otherwise Fv1 and Trim5α appear unrelated. Mutational analyses showed that restriction was invariably lost when changes designed to disrupt the sequences responsible for multimerization were introduced. A novel restriction protein could be obtained by substituting sequences from the self-associating domain of Fv1 for the Trim5 sequences in Trim5–CypA. Similarly, a fusion protein containing cyclophilin A joined to arfaptin2, a protein known to form extended dimers, was also shown to restrict HIV-1. Hence, multimerization of a capsid-binding domain could be the common minimum design feature for capsid-dependent retroviral restriction factors. However, not all domains that promote multimerization can substitute for the N-terminal domains of Fv1 and Trim5α. Moreover, only CypA can provide a capsid-binding site with different N-terminal domains. It is suggested that the spatial relationship between the multiple target binding sites may be important for restriction.