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.
Ogasawara
M
M
Pulmonary-Critical Care Medicine Branch, NHLBI, National Institutes of Health, Bethesda, Maryland 20892-1434, USA. ogawawam@gwgate.nhlbi.nih.gov
Kim
S C
SC
Adamik
R
R
Togawa
A
A
Ferrans
V J
VJ
Takeda
K
K
Kirby
M
M
Moss
J
J
Vaughan
M
M
D020727:ADP-Ribosylation Factors / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D015815:Cell Adhesion Molecules / Q000235:genetics* / Q000378:metabolism*; D002460:Cell Line; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics / Q000302:isolation & purification; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics* / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D017421:Sequence Analysis
C404511:CYTH4 protein, human; D015815:Cell Adhesion Molecules; D018076:DNA, Complementary; D020662:Guanine Nucleotide Exchange Factors; D011994:Recombinant Proteins; C100723:cytohesin-1; D020727:ADP-Ribosylation Factors