Project Summary Structural Basis of APOBEC Functions and HIV Restriction Apolipoprotein B mRNA-editing Enzyme Catalytic polypeptide (APOBEC) family of cytidine deaminases are capable of deaminating the cytidine to cause mutation to uridine on DNA or RNA. Human APOBEC deaminases have remarkable diverse cellular functions through specific targeting to the intended ssDNA or RNA through a combination of regulations including spatial and temporal distribution and substrate specificity. For example, APOBEC1 (A1) edits certain RNAs with the help of specific cofactors to regulate cholesterol metabolism; AID has important role in acquired immune response, it is required for antibody maturation including somatic hypermutation (SHM) and class switch recombination (CSR); APOBEC2 (A2) is involved in cardiac and skeletal muscle development; and APOBEC3 proteins (A3s, A3A-H) plays an important role in innate immunity for anti-viral activity, they can restrict internal and external nucleic acids (such as RNA and DNA viruses and retroelements) that poses danger to the genome integrity, which include internal retroelements as well as external retroviruses and other infectious viral pathogens, such as Hepatitis B Virus (HBV), Human Papillomavirus (HPV), and Human Immunodeficiency Virus (HIV). Among APOBEC proteins, A3G/A3F/A3D/A3H display strong anti-HIV activity, which are through deaminase-dependent and -independent mechanisms to inhibit viral replication and infection. However, retroviruses like HIV-1 can overcome the anti-HIV activity of APOBEC enzymes by its virus virulent factor (Vif) that specifically recruit cellular Cul5 E3 ligase to target these APOBECs for ubiquitination and degradation, leading to the viral infection. The deamination activity of APOBECs can also cause accidental genomic mutations, which can lead to various human diseases such as immune deficiency and cancer. Our long-term scientific goals are to understand the structural/functional relationship for APOBEC cellular functions and their anti-viral activities. Our specific aims are to understand the structural basis of APOBEC’s functions and the mechanisms that underlie nucleic acid interactions, multimerization, functional regulation, and viral restriction, with particular focuses on the double domain APOBEC3 subfamily members that have strong anti-retroelements and anti-HIV activities. The outcome of this research will provide valuable information for the fundamental molecular mechanisms of APOBEC functions and their anti viral activities, which can be used for the potential drug development to provide therapy for HIV/AIDS, immune disorders, and other diseases such as cancer.