Assignment of human B2-1 gene (D17S811E) to chromosome 17qter by PCR analysis of somatic cell hybrids and fluorescence in situ hybridization

Cytohesin-1: structure, function, and ARF activation

Mammalian cytohesins are a family of very similar guanine nucleotide-exchange proteins (GEPs) that activate ADP-ribosylation factors (ARFs). Cytohesins are multifunctional molecules comprising a Sec7 domain that is responsible for the GEP activity, a PH domain that binds specific phosphatidylinositol phosphates, and a coiled-coil domain responsible for homodimerization and interaction with other proteins. Cytohesin proteins are ubiquitous and have been implicated in several functions including cell spreading and adhesion, chemotaxis, protein trafficking, and cytoskeletal rearrangements, only some of which appear to depend on their ability to activate ARFs. Unlike the GEP activity of BIG1 and BIG2, the acceleration by cytohesins of guanine nucleotide exchange to generate active ARF-GTP is not inhibited by the fungal metabolite brefeldin, A (BFA). This chapter is concerned for the most part with cytohesin-1 and the assay of its GEP activity.

B2-1, a Sec7- and pleckstrin homology domain-containing protein, localizes to the Golgi complex

B2-1 is a human protein that contains both a Sec7 and a pleckstrin homology domain. The yeast Sec7 protein was previously shown to be involved in vesicle formation in the Golgi and endoplasmic reticulum. Recently, several groups have shown that B2-1 and highly similar proteins (e.g., ARNO, ARNO3) have varied cellular functions and subcellular locations. One of these is an association of the B2-1 Sec7 domain with the plasma membrane, binding to the cytoplasmic portion of the integrin beta2 chain (CD18) and is postulated to be involved in inside-out signaling. Other groups have shown that B2-1 and these related proteins are guanine nucleotide-exchange factors that act upon ADP ribosylation factors (ARFs) and are localized to the Golgi or plasma membrane. Here we report the subcellular localization of B2-1 protein. Interestingly, B2-1 does not localize to the plasma membrane but rather associates with a distinct Golgi complex compartment. B2-1's distribution can be disrupted by brefeldin A, a drug that rapidly disrupts the Golgi apparatus by inhibiting ARF activity. Furthermore, transient transfection of GFP-tagged B2-1 shows Golgi complex targeting. Excessive overexpression of transfected B2-1 causes partial Golgi dispersion.

Similarities in function and gene structure of cytohesin-4 and cytohesin-1, guanine nucleotide-exchange proteins for ADP-ribosylation factors

Activation of ADP-ribosylation factors (ARFs), approximately 20-kDa GTPases that are inactive in the GDP-bound form, depends on guanine nucleotide-exchange proteins (GEPs) to accelerate GTP binding. A novel ARF GEP, designated cytohesin-4, was cloned from a human brain cDNA library. Deduced amino acid sequence of the 47-kDa protein contains the same structural components present in cytohesin -1, -2, and -3, including an approximately 200-amino acid Sec7 domain with an approximately 100-residue pleckstrin homology domain near the C terminus. The Sec7 domain sequence is 77% identical to those of other cytohesins. Structures of the cytohesin-4 and cytohesin-1 genes were remarkably similar, except for an extra 3-base pair (GAG) exon present in cytohesin-1. Two mRNAs with and without the 3-base pair sequence were found in brain in different ratios for cytohesin-1, -2, and -3 but not cytohesin-4. Recombinant cytohesin-4 stimulated guanosine 5'-3-O-(thio)triphosphate binding by human ARF1 and ARF5 but not ARF6. Like other cytohesins and unlike the approximately 200-kDa ARF GEPs, it was not inhibited by brefeldin A. A cytohesin-4 mRNA of approximately 3.7 kilobases, abundant in leukocytes, was not detected in most tissues. Among separated populations of blood cells, approximately 90% of CD33(+) (monocytes), 80% of CD2(+) (NK/T), and 10-20% of CD19(+) (B) cells contained cytohesin-4 mRNA by in situ hybridization. Thus, in gene structure and brefeldin A-insensitive GEP activity, cytohesin-4 resembles other cytohesins, but its tissue distribution differs considerably, consistent with a different specific function.

Anergic T lymphocytes selectively express an integrin regulatory protein of the cytohesin family

It has been proposed that the maintenance of T cell anergy depends on the induction of negative regulatory factors. Differential display of reverse transcribed RNA was used to identify novel genes that might mediate this function in anergic Th1 clones. We report that anergic Th1 clones do indeed express a genetic program different from that of responsive T cells. Moreover, one gene, the general receptor of phosphoinositides 1 (GRP1), was selectively induced in anergic T cells. The GRP1, located in the plasma membrane, regulated integrin-mediated adhesion and was invariably associated with unresponsiveness in multiple models of anergy. T cells expressing retrovirally transduced GRP1 exhibited normal proliferation and cytokine production. However, GRP1-transduced T cells were not stable and rapidly lost GRP1 expression. Thus, although GRP1 may not directly mediate T cell anergy, it regulates cell expansion and survival, perhaps through its integrin-associated activities.

Human cts18.1 gene: chromosomal localization and PH-domain analysis

The human cts18.1 gene has high homology with the cytohesin gene family. By PCR analysis of a human monochromosomal somatic cell hybrid DNA panel, the cts18.1 gene was localized to chromosome 19. Diversity values of synonymous and nonsynonymous substitutions indicate that negative selection has occurred in the pleckstrin-homology (PH) domain of the cytohesin gene family. The phylogenetic tree calculated by the neighbor-joining method suggests that cts18.1 and cytohesin-2 genes are more closely related to each other than either of them is to the CLM-2 gene in the analysis of cDNA of the PH domain.

Complex regulation of multiple cytohesin-like genes in murine tissues and cells

Three cytohesin-like cDNA molecules were isolated from murine ES cell derived embryoid bodies. The genomic structure of one of the three, CLM2, has been determined and transcriptional variants of each were isolated from a mouse brain cDNA library. The relative expression patterns of CLM1, 2, 3 and their transcriptional alternatives were determined by RT-PCR, nucleotide sequencing and RNA blotting. Their broad distribution and cell and tissue specific expression patterns suggest complex regulation during development and in the adult.