The TITAN5 gene of arabidopsis encodes a protein related to the ADP ribosylation factor family of GTP binding proteins

The titan (ttn) mutants of Arabidopsis exhibit dramatic alterations in mito sis and cell cycle control during seed development. Endosperm development i n these mutants is characterized by the formation of giant polyploid nuclei with enlarged nucleoli. Embryo development is accompanied by significant c ell enlargement in some mutants (ttn1 and ttn5) but not others (ttn2 and tt n3). We describe here the molecular cloning of TTN5 using a T-DNA-tagged al lele. A second allele with a similar phenotype contains a nonsense mutation in the same coding region. The predicted protein is related to ADP ribosyl ation factors (ARFs), members of the RAS family of small GTP binding protei ns that regulate various cellular functions in eukaryotes. TTN5 is most clo sely related in sequence to the ARL2 class of ARF-like proteins isolated fr om humans, rats, and mice. Although the cellular functions of ARL proteins remain unclear, the ttn5 phenotype is consistent with the known roles of AR Fs in the regulation of intracellular vesicle transport.