The Cellular C1 Factor of the Herpes Simplex Virus Enhancer Complex Is a Family of Polypeptides

Abstract

The alpha/immediate early genes of herpes simplex virus are regulated by the specific assembly of a multiprotein enhancer complex containing the Oct-1 POU domain protein, the viral alpha-transinduction factor alpha TIF, (VP16, ICP25), and the C1 cellular factor. The C1 factor from mammalian cells is a heterogeneous but related set of polypeptides that interact directly with the alpha-transinduction factor to form a heteromeric protein complex. The isolation of cDNAs encoding the polypeptides of the C1 factor suggests that these proteins are proteolytic products of a novel precursor. The sequence of the amino termini of these polypeptide products indicate that the proteins are generated by site-specific cleavages within a reiterated 20-amino acid sequence. Although the C1 factor appears to be ubiquitously expressed, it is localized to subnuclear structures in specific cell types.

Molecular cloning of Drosophila HCF reveals proteolytic processing and self‐association of the encoded protein

“…One function of the spacer region in HCF-1 is to specify proteolytic processing (Wilson et al, 1993b;Kristie et al, 1995;Wilson et al, 1995b;Vogel and Kristie, 2000). To determine if the dHCF protein is also processed, we transiently transfected Drosophila SL2 cells with an expression vector encoding full-length dHCF tagged at the N-terminus with the T7 epitope and at the C-terminus with a FLAG epitope (illustrated schematically in Fig.…”

“…In mammalian HCF-1, processing occurs within each of the six active HCF PRO repeats and also within an unrelated sequence directly C-terminal to the last HCF PRO repeat (Kristie et al, 1995;Wilson et al, 1995a). For the HCF PRO repeats, the cleavage event is highly specific; alanine substitutions at 11 of the 26 residues flanking the cleavage point in HCF PRO repeat 2 abolishes cleavage activity and implies a protease activity that requires an unusually large recognition sequence or a precise secondary structure (Wilson et al, 1995a).…”

“…In addition, HCF-1 contains its own activation domain and this is required for optimal transactivation by the VP16-induced complex ( The arrangement of functional domains in human HCF-1 is shown in Figure 1A. HCF-1 is translated as a 230-300-kDa precursor and processed into N-and C-terminal subunits through autocatalytic proteolytic cleavage at a series of six HCF PRO repeats located near the center of the precursor polypeptide (Wilson et al, 1993b;Kristie et al, 1995;Wilson et al, 1995b;Vogel and Kristie, 2000). The resulting subunits remain stably associated via two matched pairs of interaction domains: termed SAS1N-SAS1C and SAS2N-SAS2C .…”

“…The SAS2 self-association domains flank the HCF PRO repeat region and have been mapped with less precision. The majority of HCF-1 molecules in actively proliferating cells have undergone processing suggesting this is the active form of the protein (Wilson et al, 1993b;Kristie et al, 1995;Wilson et al, 1995b). …”

The HCF repeat is an unusual proteolytic cleavage signal.

“…The human HCF gene directs the synthesis of a large transcript coding for a 2035-amino-acid protein with an apparent molecular mass of -300 kD (Wilson et al 199313; see also Kristie et al 1995). The most striking feature of the sequence of this protein is a set of six near-perfect 26-amino-acid repeats, referred to as HCF repeats (Wilson et al 1993b), located near the middle of the HCF open reading frame (ORF).…”