Chicken vitellogenin gene-binding protein, a leucine zipper transcription factor that binds to an important control element in the chicken vitellogenin II promoter, is related to rat DBP.

Abstract

We screened a chicken liver cDNA expression library with a probe spanning the distal region of the chicken vitellogenin II (VTGH) gene promoter and isolated clones for a transcription factor that we have named VBP (for vitellogenin gene-binding protein). VBP binds to one of the most important positive elements in the VTGH promoter and appears to play a pivotal role in the estrogen-dependent regulation of this gene. The protein sequence of VBP was deduced from a nearly full length cDNA copy and was found to contain a basic/zipper (bZIP) motif. As expected for a bZIP factor, VBP binds to its target DNA site as a dimer. Moreover, VBP is a stable dimer free in solution. A data base search revealed that VBP is related to rat DBP. However, despite the fact that the basic/hinge regions of VBP and DBP differ at only three amino acid positions, the DBP binding site in the rat albumin promoter is a relatively poor binding site for VBP. Thus, the optimal binding sites for VBP and DBP may be distinct. Similarities between the VBP and DBP leucine zippers are largely confined to only four of the seven helical spokes. Nevertheless, these leucine zippers are functionally compatible and appear to define a novel subfamily. In contrast to the bZIP regions, other portions of VBP and DBP are markedly different, as are the expression profiles for these two genes. In particular, expression of the VBP gene commences early in liver ontogeny and is not subject to circadian control.Genetic and biochemical studies have established that transcription by RNA polymerase II requires the assembly of a stable preinitiation complex over the proximal promoter region of each target gene (6,32 which a nonfunctional partner suppresses the activity of a functional partner. In addition to DNA binding domains (and, in some cases, dimerization domains), transcription factors also contain one or more transactivation domains that are required to mediate positive effects on the general transcriptional machinery. Subclasses of transactivation domains have been identified, and it is likely that other novel domains will be found as more transcription factors are cloned and analyzed. Much current work is directed at understanding how these activation domains function, and the notion that additional bridging factors may be involved has been advanced from several recent studies (reviewed in reference 25).Studies in our laboratory are focused on a molecular understanding of the estrogen-dependent and liver-specific transcriptional regulation of the chicken vitellogenin II (VTGII) gene. The estrogen-dependent aspect of this regulation was shown to be due to the presence of two upstream estrogen response elements, and the ability of the VTGII promoter to be activated by these elements was found to be cell type specific (5, 7). A linker scanner mutational analysis of the VTGII promoter using transient expression assays in chicken hepatoma (LMH) cells (18) and chicken embryo fibroblast cells revealed that this promoter has multiple positive elements as well as a negat...

Design of a C/EBP-specific, Dominant-negative bZIP Protein with Both Inhibitory and Gain-of-function Properties

“…The transactivating properties of epitope-tagged C/EBP␣ were similar to that of the unmodified protein (data not shown). The chimeric C/EBP␣-VBP (vitellogenein binding protein (36)) construct consists of C/EBP␣ sequences from the initiator methionine to Glu-316 (located at the junction of the basic region and leucine zipper) joined to the leucine zipper of VBP (Leu-278 to the COOH terminus). Construction of this chimera was accomplished with a triple ligation of the leucine zipper of VBP as a XhoI-HindIII PCR-generated fragment, a MluI-XhoI fragment of C/EBP␣ (amino acids 191-316) and the pMEX C/EBP␣ vector cut with MluI and HindIII.…”

Hlf, a novel hepatic bZIP protein, shows altered DNA-binding properties following fusion to E2A in t(17;19) acute lymphoblastic leukemia.

“…Of 57 amino acids, 41 were identical between Dbp and the E2a fusion partner in a region corresponding to the Dbp basic and leucine zipper motifs (on the basis of the corrected Dbp sequence in Drolet et al 1991;Iyer et al 1991). The novel Dbp-related protein fused to E2a in HAL-01 therefore has features of a bZIP protein and will hereafter be referred to as Hlf.…”

Design of a leucine zipper coiled coil stabilized 1.4 kcal mol−1 by phosphorylation of a serine in the e position

“…The first 16 amino acids are from 410 (Studier & Moffatt, 1986) and the remaining 80 amino acids are the C-terminus of VBP (Iyer et al, 1991) separated into heptads (a,b,c,d,e,f,g). The d or "leucine" positions of the leucine zipper are in bold type (I, L, L, L, C).…”

“…The amino acid sequence of the leucine zipper derivative of the bZIPprotein, vitellogenin binding protein (VBP) (Iyer et al, 1991) …”