Cloning And Characterization Of A Novel Rab Protein, Rab7b

Rab proteins are a family of small molecular weight GTPase of Ras superfmaily with characteristic molecular weight about 22-25 kD. Rab GTPases are expressed in almost all eukaryotic cells, ranging from yeast to drosophila and to human. They are ubiquitously or tissue-specifically expressed and function in all of cellular aspects. The Rab homologues in yeast all referred as YPT/Sec4P, while in human they are named as Rab. Rabs are generally specifically localized to the cytosolic face of distinct intracellular membranes. In order to maintain the continuity of protein transport and the integrity of organelle membrane, an intracellular organelle may possess more than two Rabs, e.g. late endosome contains Rab7 and Rab9. Rab7 mediates transport of proteins from early endosome to late endosome or fusion of early endosome with late endosome, while Rab9 mediates the retransport of M6PR from late endosome to trans-Golgi network. The GTP/GDP-binding ability and intrinsic GTPase activity of Rab proteins make them exist in Rab-GTP or Rab-GDP status. The switching of Rabs between GTP-bound (active) and GDP-bound (inactive) forms requires other regulatory proteins, including guanosine exchange factor (GEF), GDP dissociation inhibitor (GDI) and GTPase activating protein (GAP), and lays the foundations of Rab function. Rab GTPases are involved in cell differentiation, migration, endocytosis and exocytosis, cytosolic proteins transport and regulation of the transport and fusion of vesicle. In neuron cells, Rab GTPases play a crucial role in synaptic vesicle transmission and transmitter release. The execution of Rab GTPase biological functiones is also dependent on some effector proteins. Different Rab GTPases interact with different effector proteins, but different Rabs can also bind the same effector (bifunction effector), e.g. Rab4 and Rab5 both caninteract with Rabaptin-5. One Rab protein may associate with two or more effectors, suggesting the multifunctions of a Rab protein. Furthermore, more and more evidence indicate that various Rab proteins concertedly and concordantly regulate the cytosolic protein transport. At present, more than 40 Rab GTPases were found, but according to the human genome analysis, it is estimated that about 60 Rab ESTs exist totally. Therefore, the researches of Rab proteins are still far more than what have done, and the future researches will afford novel and interest methods and provide new insights into clinical diseases caused by dysfunction of Rab proteins.Leukemia is a kind of aggressive malignant tumor. So far, none satisfying therapy method has been established. In basis research, many researchers have tried to induce leukemia cell differentiation with drugs. Interesting evidences are found during the course of these researches. After leukemia cell lines HL-6 NB4 or U937 etc are stimulated by retino acid (RA), all-trans retino acid (ATRA) PMA or DMSO and so on, mRNA of some Rab proteins is up-regulated concomitantly with cell differentiation. Nishio H. and colleagues find that Rab3D mRNA is upregulated in the course of myeloid differentiation into granulocytic pathway, which suggests that human Rab3D may play a specific role in granulocytes functions, for example, exocytosis of neutrophil-specific granules or degranulation of both eosinophils and basophils. Similar phenomenons are found in other experiments. But, unfortunately, questions about which is the cause or result, and what is the relation between Rabs and cell differentiation, still need furthermore researches.Rab7b is initially found in the process of large-scale random sequencing of human DC cDNA library. By 5'-RACE, we obtain the full-length Rab7b cDNA, containing 1571bp and an open reading frame (ORF) of 600bp, which encodes a protein of 199 amino acids, and sequences of which have been deposited in GenBank under accession number AY094596. The molecular weight is predicated to be 22.5 kDa. Homology analysis indicates that Rab7bshares highest homology with Rab7, that is, 50% identity and 65% similarity. At nuc...