B-ZIP transcription factors (98) are exclusively eukaryotic proteins that bind to sequence-specific double-stranded DNA as homodimers or heterodimers to either activate or repress gene transcription (34). We have examined both of the recently published DNA sequences of the human genome (51, 95) and identified 56 genes that contain the B-ZIP motif. Three sequences were identical, giving a total of 53 unique B-ZIP domains with the potential to form 2,809 dimers. This creates the possibility for a tremendous range of transcriptional control (23, 50, 52). While significant effort has been directed at identifying dimerization partners of B-ZIP proteins, the full complement of dimerization partners remains to be elucidated. This review highlights two topics: (i) the known structural rules that regulate leucine zipper dimerization specificity and (ii) experimental data addressing mammalian B-ZIP dimerization partners.We have annotated the leucine zippers of all human B-ZIP domains, highlighting amino acids in the a, d, e, and g positions that appear critical for leucine zipper dimerization specificity. These data were used to group B-ZIP proteins into 12 families with similar dimerization properties: (i) those that strongly favor homodimerization within the family (PAR, CREB, Oasis, and ATF6), (ii) those that have the ability to both homodimerize and heterodimerize with similar affinities (C/EBP, ATF4, ATF2, JUN, and the small MAFs), and (iii) those that favor heterodimerization with other families (FOS, CNC, and large MAFs). BACKGROUND

The beta2-neuronal nicotinic acetylcholine receptor gene (CHRNB2) is a logical candidate for influencing smoking behavior and nicotine dependence. We discovered six single nucleotide polymorphisms (SNPs) in the CHRNB2 gene by surveying 15.4 kb of genomic sequence including a previously undescribed 3' untranslated region that extends 4.0 kb downstream of the coding region. One of the SNPs causes an amino acid substitution in exon 5, one occurs in the promoter region, one changes an intronic base, and three occur in the 3' untranslated region. The ethnically dependent allele frequencies and the marker-to-marker linkage disequilibrium patterns of five of these polymorphisms were determined. The SNPs were assayed in 743 individuals for whom information on smoking history and lifelong nicotine dependence was available. No significant associations of the individual markers or their haplotypes to smoking behavior or level of nicotine dependence were found.

Adipose-specific inactivation of both AP-1 and C/EBP families of B-ZIP transcription factors in transgenic mice causes severe lipoatrophy. To evaluate if inactivation of only C/EBP members was critical for lipoatrophy, A-C/EBP, a dominant-negative protein that specifically inhibits the DNA binding of the C/EBP members, was expressed in adipose tissue. For first 2 weeks after birth, aP2-A-C/EBP mice had no white adipose tissue(WAT), drastically reduced brown adipose tissue(BAT) and exhibited marked hepatic steatosis, hyperinsulinemia, and hyperlipidemia. However, WAT appeared during the third week, coinciding with significantly improved metabolic functioning. In adults, BAT remained reduced, causing cold intolerance. At 30 weeks, the aP2-A-C/EBP mice had only 35% reduced WAT, with clear morphological signs of lipodystrophy in subcutaneous fat. Circulating leptin and adiponectin levels were less than the wild type levels and these mice exhibited impaired triglyceride clearance. Insulin resistance, glucose intolerance, and reduced free fatty acid release in response to β3-adrenergic agonist suggest improper functioning of the residual WAT. Gene-expression analysis of inguinal WAT identified reduced mRNA levels of several enzymes involved in fatty acid synthesis and glucose metabolism that are known C/EBPα transcriptional targets. There were increased levels for genes involved in inflammation and muscle differentiation. However, when dermal-fibroblasts from aP2-A-C/EBP mice were differentiated into adipocytes in tissue culture, muscle markers were elevated more than the inflammatory markers. These results demonstrate that the C/EBP family is essential for adipose tissue development during the early postnatal period, contribute to glucose and lipid homeostasis in adults, and the suppression of the muscle lineage.