Much of what has been learned about HCF-1 to date is derived from investigation of its interactions with both viral and cellular transcription components. HCF-1 was originally identified and purified as a protein required for the stable assembly of the viral IE EC complex [11
]. However, the protein is now recognized as an essential cellular coactivator with global impact on gene transcription and cell cycle progression via interactions with multiple cellular transcription factors, coactivators, and chromatin modification components.
HCF-1 interacts with components that mediate both basal level and viral induced expression of the α-herpesvirus IE genes [17
]. These transcriptional activators bind various HCF-1 domains, suggesting that the protein orchestrates a coordinated regulatory process that results in the high level expression of the IE genes upon initial infection (). The viral IE activators (VP16 and ORF10) bind the amino-terminal kelch domain [18
] (). Many of the factors that bind this domain, including the viral IE activators, contain a small interaction motif (D/EXHY; referred to as the HBM, HCF binding motif) [18
]. However, despite this common motif, mutations in the HCF-1 kelch domain indicate that there are distinct binding determinants for different factors and that multiple HBM proteins may interact with the same HCF-1 molecule [23
]. In addition to the viral IE activators, the CREB/ATF family member CREB3/Luman also binds the kelch domain and can induce a representative viral IE gene in an HCF-1 dependent manner [18
]. Conversely, Zhangfei, a second bZIP protein that binds this domain can inhibit HCF-1 dependent activation of the HSV IE genes [27
] and has been hypothesized to play a role in suppression of lytic infection.
The transcriptional coactivator HCF-1 and interactions regulating the expression of the α-herpesvirus IE genes
The mid-amino terminal (MN) domain of HCF-1 interacts with GA-binding protein (GABP/NRF2) [29
], Sp1 [30
], and IE62, a second VZV IE activator that is associated with HCF-1 via Sp1 [31
]. Sp1 plays a significant role in the basal level expression of the IE genes [13
] while GABP plays a co-stimulatory role in the presence of the viral IE activator [15
]. As noted above, GABP can also promote the regulated induction of the IE genes even in the absence of the viral EC elements or the EC nucleating factor Oct-1 [16
]. Induction by the viral activators through the EC or induction via GABP are two of the multiple mechanisms by which these genes may be regulated. Notably, however, HCF-1 is required for both mechanisms suggesting that the protein mediates some common and essential rate limiting stage.
The central region of HCF-1 (proteolytic processing domain, PPD) interacts with a series of proteins including the transcriptional coactivator FHL2; a protein which functions cooperatively with HCF-1 to stimulate the HSV IE genes [32
]. Interestingly, the HCF-1 PPD consists of a series of 20 amino acid reiterations that are the sites of specific proteolytic cleavage. Processing at one or more of these sites generates a family of HCF-1 amino- and carboxy-terminal subunits that do not segregate but rather remain tightly associated [10
]. As the interaction determinants for FHL2 lie within the PPD, cleavage of HCF-1 at specific reiterations can result in the loss of FHL2 binding. Thus HCF-1 processing may represent a unique mechanism for controlling its interactions and coactivator function. Other studies have suggested that processing of HCF-1 is important in its role in modulation of cell cycle progression although the mechanism involved has not been elucidated [34
The carboxy-terminal domain of HCF-1 contains several defined functional regions including (i) a transactivation domain that synergistically functions with VP16 and other HCF-1 dependent factors, possibly through cooperative recruitment of p300 [35
]; (ii) a series of fibronectin repeats that mediate the amino- and carboxy- HCF-1 subunit association [37
]; and (iii) a bi-partite nuclear localization signal (NLS). Interestingly, transfection of an HCF-1 NLS(-) mutant results in cytoplasmic localization of both HCF-1 and VP16, suggesting that HCF-1 functions to cotransport the viral activator upon infection [38
]. However, in other studies, depletion of HCF-1 by siRNA did not prevent the nuclear accumulation of VP16, indicating that there are alternative mechanisms by which the viral activator is transported [17