Based on increasing evidence that the type I R subunits as well as the type LT:R subunits localize to specific subcellular sites, we have carried out an extensive characterization of the stable dimerization domain at the N terminus of RI alpha. Deletion mutants as well as alanine scanning mutagenesis were used to delineate critical regions as well as particular amino acids that are required for homodimerization. A set of nested deletion mutants defined a minimum core required for dimerization, Two single site mutations on the C37H template, RI alpha(F47A) and RI alpha(F52A), were sufficient to abolish dimerization. In addition to serving as a dimerization motif, this domain also serves as a docking surface for binding to dual specificity anchoring proteins (D-AKAPs) (Huang, L. J., Durick, H., Weiner, J. A., Chun, J., and Taylor, S. S. (1997) J. Biol. Chem. 272, 8057-8064; Huang, L. J., Durick, K., Weiner, J. A, Chun, J., and Taylor, S. S. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 11184-11189). A similar strategy was used to map the sequence requirements for anchoring of RI alpha to D-AKAP1. Although dimerization appears to be essential for anchoring to D-AKAP1, anchoring can also be abolished by the following single site mutations: C37H, V20A, and I25A. These sites define "hot spots" for the anchoring surface since each of these dimeric proteins are deficient in binding to D-AKAP1. In contrast to earlier predictions, the alignment of the dimerization/docking domains of RI(U and RII show striking similarities yet subtle differences not only in their secondary structure (Newlon, M. G., Boy, M., Hausken, Z. E., Scott, J. D., and Jennings, P. A. (1997) J. Biol. Chem. 272, 23637-23644) but also in the distribution of residues important for both docking and dimerization functions.
