Monomeric structure of the human EphB2 sterile alpha motif domain

The sterile a motif (SAM) domain is a protein module found in many diverse signaling proteins. SAM domains in some systems have been shown to self-ass ociate. Previous crystal structures of an EphA4-SAM domain dimer (Stapleton , D,, Balan, I., Pawson, T,, and Sicheri, F, (1999) Not, Struct. Biol. 6, 4 4-49) and a possible EphB2-SAM oligomer (Thanos, C, D,, Goodwill, K, E,, an d Bowie, J, U, (1999) Science 283, 833-836) both revealed large interfaces comprising an exchange of N-terminal peptide arms. Within the arm, a conser ved hydrophobic residue (Tyr-8 in the EphB2-SAM structure or Phe-910 in the EphA4-SAM structure) is anchored into a hydrophobic cleft on a neighboring molecule. Here we have solved a new crystal form of the human EphB2-SAM do main that has the same overall SARI domain fold yet has no substantial inte rmolecular contacts. In the new structure, the N-terminal peptide arm of th e EphB2-SAM domain protrudes out from the core of the molecule, leaving bot h the arm (including Tyr-8) and the hydrophobic cleft solvent-exposed, To v erify that Tyr-8 is solvent-exposed in solution, we made a Tyr-8 to Ala-8 m utation and found that the EphB2-SAM domain structure and stability were on ly slightly altered. These results suggest that Tyr-8 is not part of the hy drophobic core of the EphB2-SAM domain and is conserved for functional reas ons. Cystallographic evidence suggests a possible role for the N-terminal a rm in oligomerization, In the absence of a direct demonstration of biologic al relevance, however, the functional role of the N-terminal arm remains an open question.