Ra1BP1, a multifunctional protein implicated in cancer cell proliferation, radiation and chemo resistance, and ligand dependent receptor internalization, is upregulated in bladder cancer and is a downstream effector of RalB, a GTPase associated with metastasis. RaIBPI can be regulated by phosphorylation by protein kinase C (PKC). No studies have comprehensively mapped Ra1BP1 phosphorylation sites or whether RalB affects these. We identified 14 phosphorylation sites of Ra1BP1 in human bladder carcinoma UMUC-3 and embryonic kidney derived 293T cells. The phosphorylated residues are concentrated at the N-terminus. Ten of the first 100 amino acids of the primary structure were phosphorylated. Nine were serine residues, and one a threonine. We evaluated the effect of Ra1B overexpression on Ra1BP1 phosphorylation and found the largest change in phosphorylation status at S463 and S645. Further characterization of these sites will provide novel insights on RaIBPI biology, its functional relationship to RalB and possible avenues for therapeutic intervention. (c) 2007 Elsevier Inc. All rights reserved.