A novel RalGEF-like protein, RGL3, as a candidate effector for Rit and Ras

The small GTPase Rit is a close relative of Ras, and constitutively active Rit can induce oncogenic transformation. Although the effector loops of Rit and Ras are highly related, Rit fails to interact with the majority of the known Ras candidate effector proteins, suggesting that novel cellular targ ets may be responsible for Rit transforming activity. To gain insight into the cellular function of Rit, we searched for Rit-binding proteins by yeast two-hybrid screening. We identified the C-terminal Rit/Ras interaction dom ain of a protein we have designated RGL3 (Ral GEF-like 3) that shares 35% s equence identity with the known Ral guanine nucleotide exchange factors (Ra lGEFs). RGL3, through a C-terminal 99-amino acid domain, interacted in a GT P- and effector loop-dependent manner with Rit and Ras. Importantly, RGL3 e xhibited guanine nucleotide exchange activity toward the small GTPase Ral t hat was stimulated in vivo by the expression of either activated Rit or Res . These data suggest that RGL3 functions as an exchange factor for Ral and may serve as a downstream effector for both Rit and Ras.