HIV-1-induced translocation of CPSF6 to biomolecular condensates

Abstract

Cleavage and polyadenylation specificity factor subunit 6 (CPSF6, also known as CFIm68) is a 68 kDa component of the mammalian cleavage factor I (CFIm) complex that modulates mRNA alternative polyadenylation (APA) and determines 3' untranslated region (UTR) length, an important gene expression control mechanism. CPSF6 directly interacts with the HIV-1 core during infection, suggesting involvement in HIV-1 replication. Here, we review the contributions of CPSF6 to every stage of the HIV-1 replication cycle. Recently, several groups described the ability of HIV-1 infection to induce CPSF6 translocation to nuclear speckles, which are biomolecular condensates. We discuss the implications for CPSF6 localization in condensates and the potential role of condensate-localized CPSF6 in the ability of HIV-1 to control the protein expression pattern of the cell.

Keywords: CPSF6; HIV-1; alternative polyadenylation; biomolecular condensates; nuclear speckles.

Figure 1.. Protein domain organization of cleavage and polyadenylation specificity factor subunit 6 (CPSF6).

The various domains and functions of human CPSF6. The amino acid (aa) positions for each large domain are shown: RNA-recognition motif in orange (aa 81–157); proline-rich domain in yellow (aa 208–398); and arginine-serine-like domain (RSLD) in red (aa 489–551). Additional interaction motifs are shown in green: CPSF5 interaction motif (aa 116–122), glycine/arginine-rich domain (aa 202–206), the HIV-1 capsid interaction motif (aa 276–290), and the transportin 1 (TNPO1) interaction motif (aa 362–390).

Figure 2.. Cleavage and polyadenylation specificity factor subunit 6 (CPSF6) is a component of the mammalian cleavage factor I (CFIm) complex.

(A) CPSF6’s role in alternative polyadenylation. CPSF6, a component of the CFIm complex, is involved in the maturation of pre-mRNA into functional mRNA. CPSF6 controls alternative polyadenylation by selecting the polyadenylation signal. Cleavage and polyadenylation are controlled by upstream (UGUA, hexameric A[A/U]UAAA, and U-rich) and downstream (GU-rich and U-rich) cis-elements. UGUA is recognized by the CFIm complex (CPSF5₂ and either CPSF6₂ or CPSF7₂). CPSF5 binds directly to the UGUA element and anchors CPSF6 to the transcript. CPSF6 positions the complex at the appropriate cleavage and polyadenylation site and enhances RNA binding. CFIm complex interacts with the CFIIm complex, which is required for RNA cleavage [82]. The A[A/U]UAAA motif recruits the CPSF complex to promote cleavage stimulation factor (CstF). After cleavage the 3′ end of the transcript is then subjected to poly(A) tail addition by poly(A) polymerase. (B) Alternative polyadenylation. Human genes frequently encode more than one polyadenylation signal. The transcript isoforms that are derived from a single gene with two polyadenylation signals (PASs) are illustrated. When the gene uses the proximal PAS, the mature mRNA contains a short 3’UTR. By contrast, the use of a distal PAS results in an mRNA with a long 3’UTR. The ability of the CFIm complex to use the proximal polyadenylation signal instead of the distal signal is referred to as alternative polyadenylation. GOI, gene of interest.

Figure 3.. HIV-1 infection induces translocation of cleavage and polyadenylation specificity factor subunit 6 (CPSF6) and CPSF5 to nuclear speckles.

The cellular changes observed in CPSF6, CPSF5, and lens epithelium–derived growth factor (LEDGF)/p75 localization following HIV-1 infection are illustrated. In uninfected cells, CPSF6, CPSF5, and LEDGF/p75 are distributed throughout the nucleus (left). Upon HIV-1 infection (right), CPSF6 and CPSF5 are translocated into nuclear speckles, subnuclear structures without membranes that behave like biomolecular condensates. LEDGF/p75 appears to surround nuclear speckles following HIV-1 infection (right).