Arf proteins bind to mitotic kinesin-like protein 1 (MKLP1) in a GTP- dependent fashion

Arf proteins comprise a family of 21-kDa GTP-binding proteins with many proposed functions in mammalian cells, including the regulation of several steps of membrane transport, maintenance of organelle integrity, and activation of phospholipase D. We performed a yeast two-hybrid screen of human cDNA libraries using a dominant activating allele, [Q71L], of human Arf3 as bait. Eleven independent isolates contained plasmids encoding the C- terminal tail of mitotic kinesin-like protein-1 (MKLP1). Further deletion mapping allowed the identification of an 88 amino acid Arf3 binding domain in the C-terminus of MKLP1. This domain has no clear homology to other Arf binding proteins or to other proteins in the protein databases. The C- terminal domain of MKLP1 was expressed and purified from bacteria as a GST fusion protein and shown to bind Arf3 in a GTP-dependent fashion. A screen for mutations in Arf3 that specifically lost the ability to bind MKLP1 identified 10 of 14 point mutations in the GTP-sensitive switch I or switch II regions of Arf3. Two-hybrid assays of the C-terminal domain of MKLP1 with each of the human Arf isoforms revealed strong interaction with each. Taken together, these data are all supportive of the conclusion that activated Arf proteins bind to the C-terminal 'tail' domain of MKLP1.