Genomic organization and expression of the alpha subunit of the component B of the human Rab geranylgerany transferase

초록

In order to study the promoter region of the human transglutaminase 1 gene (TGM 1) we cloned and sequenced a genomic clone containing 7.4 kb upstream of the TGM1 cap site. The comparison of this sequence with the sequences collected in Genbank revealed the presence of another gene encoding the subunit alpha of the component B of the Rab geranylgeranyl transferase (Rab gglb alpha) in close proximity with TGM1. Rab ggl is an enzyme that attaches 20-carbon geranylgeranyl groups to cysteine residues In proteins. Rab ggt consists of two components: component A (95-kDa); component B, a heterodimer formed by an alpha subunit (60-kDa) and a Beta subunit (38 kDa). The function of Rab ggt is not completely known but mutations of the component A causes choroideremia (an X-linked form of retinal degeneration). Mutation studies conducted on the yeast analogues of the human genes have shown that the alpha subunit of the component B is import ant for mitotic regulation . The last exon of Rab ggtb alpha ends 2.3 kb from the cap site of TGM1 . The observation that these two genes are so closely associated may mean they are coexpressed and functionally related. To begin to investigate this possibility we have determined the genomic structure of Rab ggtb alpha gene. The clone we have sequenced contains 13 exons and 12 introns. We then completed the genomic structure by amplifying an additional 1kb of genomic DNA containing the cDNA sequence missing from our original clone and 2 additional introns. The Rab gglb alpha gene is therefore formed by 15 exons and 14 introns spanning about 5.8 kb. To test whether Rab ggtb alpha gene and TGM1 are coexpressed and functionally related we have studied the expression of rab ggtb alpha in human foreskin epidermis and in cultured keratinocytes under various differentiation conditions. By RT-PCR, we have found this gene is indeed expressed and regulated during differentiation in the epidermis.