The human immunodeficiency virus type 1 Rev protein shuttles between the cytoplasm and nuclear compartments.

Abstract

A retroviral regulatory protein, Rev (regulator of virion protein expression), is made in cells infected by human immunodeficiency virus (11). Rev is essential for the completion of the retroviral life cycle and interacts with the host cell at some posttranscriptional step in order to express the incompletely spliced HIV mRNAs from which HIV structural proteins are translated. Neither the host cell components nor the mechanisms responsible for this important regulation have been defined. We now report that Rev is a nucleocytoplasmic shuttle protein which is continuously transported between the cytoplasm, the nucleoli, and nucleoplasmic speckles enriched in RNA splicing and processing factors. The results show that Rev has the potential to interfere specifically with the splicing of the HIV pre-mRNA in the nucleoplasm and, next, guide such mRNAs to the cytoplasm for translation.The genetic information of human immunodeficiency virus (HIV), which causes AIDS, is contained in a 9-kb genome. Elaborate differential splicing of the full-length initial transcript places the various reading frames into position for subsequent translation, resulting in three size classes of HIV mRNAs (about 9, 4, and 2 kb) (20,23,41). The two larger-size classes of HIV mRNA, which are incompletely spliced, cannot be expressed as Gag/Pol and Env proteins, respectively, unless an HIV regulatory protein, Rev, is present (16,35,44,47,50). Rev itself and Tat (11), another essential HIV protein, are made from the doubly spliced mRNA species which are expressed independently of Rev. When HIV transcription is activated in the infected cell, the full-length mRNA is spliced two times by default and translated into Tat and Rev proteins. Tat boosts transcription from the HIV promoter by positive feedback so that larger amounts of Tat and Rev are made. When Rev accumulates above a threshold level in the cell, the unspliced mRNAs (gagipol) and the singly spliced mRNAs (env) become preferentially expressed (38). The HIV life cycle thus consists of an early phase characterized by increasing Tat and Rev production from doubly spliced mRNAs and a late phase with favored expression of viral structural proteins from unspliced and singly spliced HIV mRNAs (28). The hallmark of the Rev-dependent HIV mRNAs is that they contain two types of cis-acting sequences, the Rev response element (RRE) and several cis-acting repressive sequences (42). The RRE is a sequence of 234 ribonucleotides present in the env region of the HIV mRNA between the two exons of Tat and Rev (13,36). The RRE folds into a complex stem-and-loop structure in vitro and binds avidly and specifically to the basic domain of the Rev protein (reviewed in references 11 and 23). The cis-acting repressive sequences are less well characterized but are present both in the gag/pol region and in the env region (10,32,42,46 (15,43). Apparently discrepant results have identified the host cell RNA splicing machinery, the mRNA transport system, or cytoplasmic events of mRNA expression as important targets fo...

The protein that binds the 3' end of histone mRNA: a novel RNA-binding protein required for histone pre-mRNA processing.

“…Unlike the PABP, the SLBP also participates in the nuclear metabolism of pre-mRNA, and is required for efficient histone pre-mRNA processing (Dominski et al 1995). Thus, it joins a growing family of pro-teins, including heterogenous nuclear ribonucleoprotein (hnRNP) A1 (Pifiol-Roma and Dreyfuss 1992;Michael et al 1995) and rev (Kalland et al 1994), w h i c h may bind to pre-mRNA and t h e n a c c o m p a n y the mature m R N A to the cytoplasm.…”

hRIP, a cellular cofactor for Rev function, promotes release of HIV RNAs from the perinuclear region

“…The presence of both a nuclear localization signal (NLS) and an NES enables Rev to shuttle continuously between the nucleus and cytoplasm, an activity required for its function (Kalland et al 1994;Meyer and Malim 1994;Richard et al 1994). In view of this requirement, we examined whether hRIP⌬N360 could interfere with the general NLS-dependent protein import or NES-dependent protein export pathways.…”

Nuclear and nucleolar targeting of human ribosomal protein S25: Common features shared with HIV-1 regulatory proteins

“…The functional signi®cance of the nucleolar localization of these proteins is still controversial, although a possible role of the nucleolus as a storage site for the proteins is suggested in the case of the HIV-1 Rev protein (Kubota et al, 1996), which shuttles between the cytoplasm and the nucleus, to enable the nuclear export of unspliced viral mRNAs (Malim et al, 1989a,b;Fischer et al, 1994;Kalland et al, 1994;Meyer and Malim, 1994;Richard et al, 1994). Despite its evident functional signi®cance as a pre-event of ribosomal assembly, nucleolar targeting of human ribosomal proteins has been characterized in only a few members, S6, S14 and L7a (Schmidt et al, 1995;Martin-Nieto and Roufa, 1997;Russo et al, 1997).…”