title: Molecular basis of sidekick-mediated cell-cell adhesion and specificity.
abstract: Sidekick (Sdk) 1 and 2 are related immunoglobulin superfamily cell adhesion proteins required for appropriate synaptic connections between specific subtypes of retinal neurons. Sdks mediate cell-cell adhesion with homophilic specificity that underlies their neuronal targeting function. Here we report crystal structures of Sdk1 and Sdk2 ectodomain regions, revealing similar homodimers mediated by the four N-terminal immunoglobulin domains (Ig1-4), arranged in a horseshoe conformation. These Ig1-4 horseshoes interact in a novel back-to-back orientation in both homodimers through Ig1:Ig2, Ig1:Ig1 and Ig3:Ig4 interactions. Structure-guided mutagenesis results show that this canonical dimer is required for both Sdk-mediated cell aggregation (viatransinteractions) and Sdk clustering in isolated cells (viacisinteractions). Sdk1/Sdk2 recognition specificity is encoded across Ig1-4, with Ig1-2 conferring the majority of binding affinity and differential specificity. We suggest that competition betweencisandtransinteractions provides a novel mechanism to sharpen the specificity of cell-cell interactions.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States.
authors.forename: Kerry M
authors.initials: KM
authors.lastname: Goodman
authors.affiliation: Department of Molecular and Cellular Biology, Harvard University, Cambridge, United States.|Center for Brain Science, Harvard University, Cambridge, United States.
authors.forename: Masahito
authors.identifier: 0000-0001-8193-2931
authors.initials: M
authors.lastname: Yamagata
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States.|Howard Hughes Medical Institute, Columbia University, New York, United States.
authors.forename: Xiangshu
authors.initials: X
authors.lastname: Jin
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States.
authors.forename: Seetha
authors.initials: S
authors.lastname: Mannepalli
authors.affiliation: Howard Hughes Medical Institute, Columbia University, New York, United States.|Department of Systems Biology, Columbia University, New York, United States.
authors.forename: Phinikoula S
authors.initials: PS
authors.lastname: Katsamba
authors.affiliation: Howard Hughes Medical Institute, Columbia University, New York, United States.|Department of Systems Biology, Columbia University, New York, United States.
authors.forename: Göran
authors.initials: G
authors.lastname: Ahlsén
authors.affiliation: Howard Hughes Medical Institute, Columbia University, New York, United States.|Department of Systems Biology, Columbia University, New York, United States.
authors.forename: Alina P
authors.initials: AP
authors.lastname: Sergeeva
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States.|Howard Hughes Medical Institute, Columbia University, New York, United States.|Department of Systems Biology, Columbia University, New York, United States.|Department of Medicine, Columbia University, New York, United States.|Zuckerman Mind Brain and Behavior Institute, Columbia University, New York, United States.
authors.forename: Barry
authors.identifier: 0000-0002-2480-6696
authors.initials: B
authors.lastname: Honig
authors.affiliation: Department of Molecular and Cellular Biology, Harvard University, Cambridge, United States.|Center for Brain Science, Harvard University, Cambridge, United States.
authors.forename: Joshua R
authors.identifier: 0000-0001-8926-8836
authors.initials: JR
authors.lastname: Sanes
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Columbia University, New York, United States.|Department of Systems Biology, Columbia University, New York, United States.|Zuckerman Mind Brain and Behavior Institute, Columbia University, New York, United States.
authors.forename: Lawrence
authors.identifier: 0000-0001-9943-8819
authors.initials: L
authors.lastname: Shapiro
mesh_terms: D002448:Cell Adhesion; D018360:Crystallography, X-Ray; D004252:DNA Mutational Analysis; D007074:Immunoglobulin G / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D009474:Neurons / Q000502:physiology*; D011487:Protein Conformation; D055503:Protein Multimerization; D012160:Retina / Q000502:physiology*
chemical_list: D007074:Immunoglobulin G; D008565:Membrane Proteins; D050505:Mutant Proteins; C506156:Sdk2 protein, mouse; C493090:sidekick-1 protein, mouse
keywords: biophysics; cell adhesion; crystal structure; homophilic specificity; mouse; neural patterning; neuroscience; structural biology; synaptic targeting