Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:2.7.7.49 (reverse transcriptase)
 31,746 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The APOBEC3 restriction factors are a family of deoxycytidine deaminases that are able to suppress replication of viruses with a single-stranded DNA intermediate by inducing mutagenesis and functional inactivation of the virus. Of the seven human APOBEC3 enzymes, only APOBEC3-D, -F, -G, and -H appear relevant to restriction of HIV-1 in CD4+ T cells and will be the focus of this review. The restriction of HIV-1 occurs most potently in the absence of HIV-1 Vif that induces polyubiquitination and degradation of APOBEC3 enzymes through the proteasome pathway. To restrict HIV-1, APOBEC3 enzymes must be encapsidated into budding virions. Upon infection of the target cell during reverse transcription of the HIV-1 RNA into (-)DNA, APOBEC3 enzymes deaminate cytosines to form uracils in single-stranded (-)DNA regions. Upon replication of the (-)DNA to (+)DNA, the HIV-1 reverse transcriptase incorporates adenines opposite to the uracils thereby inducing C/G to T/A mutations that can functionally inactivate HIV-1. APOBEC3G is the most studied APOBEC3 enzyme and it is known that Vif attempts to thwart APOBEC3 function not only by inducing its proteasomal degradation but also by several degradation-independent mechanisms, such as inhibiting APOBEC3G virion encapsidation, mRNA translation, and for those APOBEC3G molecules that still become virion encapsidated, Vif can inhibit APOBEC3G mutagenic activity. Although most Vif variants can induce efficient degradation of APOBEC3-D, -F, and -G, there appears to be differential sensitivity to Vif-mediated degradation for APOBEC3H. This review examines APOBEC3-mediated HIV restriction mechanisms, how Vif acts as a substrate receptor for a Cullin5 ubiquitin ligase complex to induce degradation of APOBEC3s, and the determinants and functional consequences of the APOBEC3 and Vif interaction from a biological and biochemical perspective.
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PMID:Suppression of APOBEC3-mediated restriction of HIV-1 by Vif. 2520 52

The APOBEC3 family of deoxycytidine deaminases has the ability to restrict HIV-1 through deamination-dependent and deamination-independent mechanisms. Although the generation of mutations through deamination of cytosine to uracil in single-stranded HIV-1 (-) DNA is the dominant mechanism of restriction, the deaminase-independent mechanism additionally contributes. Previous observations indicate that APOBEC3 enzymes competitively bind the RNA template or reverse transcriptase (RT) and act as a roadblock to DNA polymerization. Here we studied how the deamination-independent inhibition of HIV-1 RT by APOBEC3C S188I, APOBEC3F, APOBEC3G, and APOBEC3H affected RT template switching. We found that APOBEC3F could promote template switching of RT, and this was dependent on the high affinity with which it bound nucleic acids, suggesting than an APOBEC3 "road-block" can force template switching. Our data demonstrate that the deamination-independent functions of APOBEC3 enzymes extend beyond only disrupting RT DNA polymerization. Since alterations to the RT template switching frequency can result in insertions or deletions, our data support a model in which APOBEC3 enzymes use multiple mechanisms to increase the probability of generating a mutated and nonfunctional virus in addition to cytosine deamination.
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PMID:APOBEC3 Host Restriction Factors of HIV-1 Can Change the Template Switching Frequency of Reverse Transcriptase. 3079 59

APOBEC3 proteins APOBEC3F (A3F), APOBEC3G (A3G), and APOBEC3H (A3H) are host restriction factors that inhibit HIV-1 through DNA cytidine deaminase-dependent and -independent mechanisms and have either one (A3H) or two (A3F and A3G) zinc-binding domains. A3H antiviral activity encompasses multiple molecular functions, all of which depend on recognition of RNA or DNA. A3H crystal structures revealed an unusual interaction with RNA wherein an RNA duplex mediates dimerization of two A3H proteins. In this study, we sought to determine the importance of RNA-binding amino acids in the antiviral and biochemical properties of A3H. We show that the wild-type A3H-RNA interaction is essential for A3H antiviral activity and for two deaminase-independent processes: encapsidation into viral particles and inhibition of reverse transcription. Furthermore, an extensive mutagenesis campaign revealed distinct roles for two groups of amino acids at the RNA binding interface. C-terminal helix residues exclusively bind RNA, and loop 1 residues play a dual role in recognition of DNA substrates and in RNA binding. Weakening the interface between A3H and RNA allows DNA substrates to bind with greater affinity and enhances deamination rates, suggesting that RNA binding must be disrupted to accommodate DNA. Intriguingly, we demonstrate that A3H can deaminate overhanging DNA strands of RNA/DNA heteroduplexes, which are early intermediates during reverse transcription and may represent natural A3H substrates. Overall, we present a mechanistic model of A3H restriction and a step-by-step elucidation of the roles of RNA-binding residues in A3H activity, particle incorporation, inhibition of reverse transcriptase inhibition, and DNA cytidine deamination.IMPORTANCE APOBEC3 proteins are host factors that protect the integrity of the host genome by inhibiting retroelements as well as retroviruses, such as HIV-1. To do this, the APOBEC3H protein has evolved unique interactions with structured RNAs. Here, we studied the importance of these interactions in driving antiviral activity of APOBEC3H. Our results provide a clear picture of how RNA binding drives the ability of APOBEC3H to infiltrate new viruses and prevent synthesis of viral DNA. We also explore how RNA binding by APOBEC3H influences recognition and deamination of viral DNA and describe two possible routes by which APOBEC3H might hypermutate the HIV-1 genome. These results highlight how one protein can sense many nucleic acid species for a variety of antiviral activities.
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PMID:Flexibility in Nucleic Acid Binding Is Central to APOBEC3H Antiviral Activity. 3157 94