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Comparative Study
. 2004 Mar 9;101(10):3468-73.
doi: 10.1073/pnas.0400268101. Epub 2004 Feb 26.

Basonuclin 2: an extremely conserved homolog of the zinc finger protein basonuclin

Amandine Vanhoutteghem  1 , Philippe Djian
Affiliations
Comparative Study

Basonuclin 2: an extremely conserved homolog of the zinc finger protein basonuclin

Amandine Vanhoutteghem et al. Proc Natl Acad Sci U S A. .

Abstract

Basonuclin is a zinc finger protein specific to basal keratinocytes and germ cells. In keratinocytes, basonuclin behaves as a stem cell marker and is thought to be a transcription factor that maintains proliferative capacity and prevents terminal differentiation. The human gene is located on chromosome 15. We have discovered in the chicken the existence of basonuclin 2, a basonuclin homolog. We also report the entire sequence of mouse and human basonuclin 2; the corresponding genes are located on mouse chromosome 4 and human chromosome 9. Although the amino acid sequence of basonuclin 2 differs extensively from that of basonuclin 1, the two proteins share essential features. Both contain three paired zinc fingers, a nuclear localization signal, and a serine stripe. The basonuclin 2 mRNA has a wider tissue distribution than the basonuclin 1 mRNA: it is particularly abundant in testis, kidney, uterus, and intestine. The extreme conservation of the basonuclin 2 amino acid sequence across vertebrates suggests that basonuclin 2 serves an important function, presumably as a regulatory protein of DNA transcription.

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Figures

Fig. 5.
Fig. 5.
Alignment of the amino acid sequences of basonuclin 2 of various species. The deduced amino acid sequences of the entire human and mouse basonuclin 2 have been aligned with partial sequences of the chicken and zebrafish. Divergent residues are boxed. The protein is practically identical in all species examined. Mouse sequence (Mobn2) was from GenBank entry NM_172870. The human sequence (Hubn2) was assembled from exons found on BACs AL450105, AL450003, and AL449983 (see Fig. 7). The chicken sequence (Chbn2) was derived from EST 603508405F1 (http://chick.umis-t.ac.uk). The zebrafish sequence (Zfbn2) was from two exons on BAC BX000462.
Fig. 1.
Fig. 1.
Alignment of the amino acid sequences of mouse basonuclin 1 and 2. The main features of the basonuclin 2 sequence (Mobn2) are the six zinc fingers (underlined) with their cysteines and histidines (circled), the NLS (bold type), and the serine stripe (framed). These features are also found in basonuclin 1 (Mobn1). The basonuclin 1 sequence is from Matsuzaki et al. (9). The middle sequence corresponds to residues shared by basonuclin 1 and 2. +, Conservative amino acid replacement.
Fig. 2.
Fig. 2.
Map of the mouse basonuclin 2 gene. Exons are represented by black boxes; they are designated by Arabic numerals. Introns are indicated by Roman numerals. Introns are not drawn to scale. The regions of interest of each of the six exons are assigned.
Fig. 3.
Fig. 3.
Comparison of intron boundaries of basonuclin 1 and 2. Numbers in parentheses indicate the amino acid position of mouse basonuclin 2 (see Fig. 1) at which intron interrupt exons.
Fig. 4.
Fig. 4.
Alternative splicing of basonuclin 2 gene. The accession numbers of the various mouse and human mRNAs found in GenBank are given. The asterisk indicates an error in the sequence of HSM806660 reported in GenBank. An adenine between nucleotides 1254 and 1255 is missing; this adenine is found on BAC AL449983. The primary transcript can be spliced in different ways (introns are designated with Roman numerals) and translated into different proteins, some of which lack all or some of the zinc fingers (thick vertical bars in the protein). Regions of proteins shared with basonuclin 2 (NM_172870) are shaded, whereas regions that differ are cross-hatched. UTRs are empty. Dashed lines show that some protein sequences are incomplete because they are deduced from partial-length cDNAs. AK031440 contains two additional exons (1a and 2a). Exon 1a encodes the putative initiating methionine; exon 2a is in frame with exons 2 and 4. The AK031440 coding region terminates within intron IV. AK000050, AK025882, and BC037160 are similar to NM_172870. AK092247 contains two additional small exons (5a and 5b) at its 3′ end and terminates within exon 5a. It therefore lacks zinc fingers 4–6. HSM806660 uses a poly(A) addition site located 2 kb downstream from that used by the other human transcripts. It encodes the usual protein. Slashes indicate that exon 6 of HSM806660 is not drawn to scale.
Fig. 6.
Fig. 6.
Presence of basonuclin 2 mRNA in various tissues. (A) A Northern blot containing 2 μg of poly(A)+ RNA prepared from various human tissues was purchased from Clontech. RNAs were successively hybridized to a 733-bp basonuclin 2-specific probe (Left), an 802-bp basonuclin 1 probe (Right), and a human actin probe. Testis, uterus, and intestine gave appreciable signals for basonuclin 2, prostate gave a weak signal, and colon gave a trace signal. All other tissues were negative. The human basonuclin 2 mRNA was estimated at 6 kb. Testis was the only tissue that gave a signal for basonuclin 1; two transcripts at 4.6 and 3.2 kb were present, as described in ref. . The sources of the RNAs are indicated. (B) RT-PCR analysis of total RNA prepared from various adult mouse tissues by using oligonucleotide primers designed to amplify specifically an 894-bp basonuclin 2 fragment and an 810-bp basonuclin 1 fragment. The PCR products were resolved on a 1% agarose gel and visualized by ethidium bromide staining. The mouse basonuclin 2 mRNA is found in all tissues examined. It is particularly abundant in testis and kidney and less abundant in epidermis and small intestine. Liver and colon contain trace amounts (the image is overexposed). The basonuclin 1 mRNA is only found in testis and epidermis, in which it is less abundant than the basonuclin 2 mRNA. (C) RT-PCR analysis of total RNA prepared from human cultured keratinocytes. The mRNA for basonuclin 1 (802-bp fragment) is ≈10 times more abundant than that for basonuclin 2 (733-bp fragment).
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