Evolutionary conservation and genomic organization of XAP-4, an Xq28 located gene coding for a human rab GDP-dissociation inhibitor (GDI).

After the development of efficient methods for the construction of transcription maps of defined genomic regions, the rate-limiting step in the analysis of the coding potentials of these regions is the elucidation of function of the novel genes and the examination of their possible involvement in hereditary diseases localized to the region. This can be greatly facilitated by the detection of sequence homology to a gene of known function. XAP-4 is one of the genes identified in the G6PD region of the human Xq28 by direct cDNA selection. The rapid assembly of this gene and the determination of its function was possible because of its sequence homology with the bovine smg p25A/rab3A GDP dissociation inhibitor (GDI). Sequence comparison with other GDIs in the databases has revealed that XAP-4 belongs to one of at least two distinct classes of mammalian rab GDIs. The rab GDIs, which play an important role in the regulation of cellular transport, are highly evolutionarily conserved, as are several other genes identified in the neighborhood of XAP-4. This genomic region is very gene dense, and all the cDNA clones from the approximately 2.5-kb-long transcript of XAP-4 map to a single 7.5-kb genomic EcoRI fragment. The genomic organization of XAP-4 has been examined to determine the distribution of the exonic sequences within this short segment of genomic DNA. It was found that, similar to several other genes from the region, XAP-4 is split into exons of average size, which are interrupted by very short introns.