Chromosomal mapping, gene structure and characterization of the human and murine RAB27B gene

The human RAB27B gene is organised in six exons, spanning about 69 kb in the chromosome 18q21.1 region. Exon 1 is non-coding and is separated from the others by 49 kb of DNA and exon 6 contains a long 3' untranslated sequence (6.4 kb). The mouse Rab27b cDNA shows 95% identity with the human cDNA at the protein level and maps to mouse chromosome 18. The mouse mRNA was detected in stomach, large intestine, spleen and eye by RT-PCR, and in heart, brain, spleen and kidney by Northern blot. Transient over-expression of EGF-Rab27b fusion protein in cultured melanocytes revealed that Rab27b is associated with melanosomes, as observed for EGF-Rab27a.Our results indicate that the Rab27 subfamily of Ras-like GTPases is highly conserved in mammals. There is high degree of conservation in sequence and gene structure between RAB27A and RAB27B genes. Exogenous expression of Rab27b in melanocytes results in melanosomal association as observed for Rab27a, suggesting the two Rab27 proteins are functional homologues. As with RAB27A in Griscelli Disease, RAB27B may be also associated with human disease mapping to chromosome 18.Rab proteins constitute a large group within the Ras superfamily of low molecular weight GTPases. Rab GTPases regulate vesicular transport steps in endocytic and exocytic pathways, acting as molecular switches oscillating between active GTP-bound and inactive GDP-bound states [1,2]. Over 50 different Rab proteins have been identified to date in mammalian cells while the yeast S. cerevisae has 11 Rabs (termed Ypt and Sec4) [3,4]. Within this family of proteins, most members share about 35% identity, while there are some Rabs that show unusually high identity (more than 70%) and are considered isoforms, eg, Rab1a and Rab1b.The Rab27 subfamily consists of Rab27a (previously designated Ram) [5] and Rab27b (previously designated c25KG) [6]. Rab27a was initially cloned from a megakaryocyte library and subsequent studies revealed that it is expressed in haemopoietic-de